The Student Room Group

Please help me with reacting mass questions

Hello,

I have been searching all over the internet to find help but no matter what, I just can't understand it. I have this long list of equations which I need to complete for homework and I am struggling on the first question.

Please can you help me with this question: what mass of potassium oxide is formed when 7.8g of potassium is burned in oxygen?

4K + O2 -> 2K2O

Please can you show me how you work it out please and, if possible, explain each step.

Many thanks :smile:
Reply 1
Original post by muffin_selight
Hello,

I have been searching all over the internet to find help but no matter what, I just can't understand it. I have this long list of equations which I need to complete for homework and I am struggling on the first question.

Please can you help me with this question: what mass of potassium oxide is formed when 7.8g of potassium is burned in oxygen?

4K + O2 -> 2K2O

Please can you show me how you work it out please and, if possible, explain each step.

Many thanks :smile:


Do you have the Nelson Thornes textbook? It explains it fairly clearly in there, using it follow the steps and show what you get then we could point you in the right direction :biggrin:
Original post by Secret.
Do you have the Nelson Thornes textbook? It explains it fairly clearly in there, using it follow the steps and show what you get then we could point you in the right direction :biggrin:


No, I have an OCR one, it doesn't explain the calculations very well :s-smilie:
Reply 3
Original post by muffin_selight
No, I have an OCR one, it doesn't explain the calculations very well :s-smilie:



Aah okay well try follow these steps:

1) Make sure equation is balanced
2) Find the number of moles of each element that you want to know
3) Take a ratio of the balanced equations of the moles
4) Turn moles back into what you want

so try that with the question you have :smile:
7.8/32 = moles of O2. Moles of O2 x 4 = moles of K. Moles of K x Ar of K = mass

This was posted from The Student Room's Android App on my GT-S5830i
Original post by PKMN TRN JAM
7.8/32 = moles of O2. Moles of O2 x 4 = moles of K. Moles of K x Ar of K = mass

This was posted from The Student Room's Android App on my GT-S5830i


Sorry but I don't understand why you are using oxygen? I am still confused. This is what I got for this question:

7.8/(39.1x4)=0.5 moles
0.5/2 (because of the 2:1 ratio)=0.25
0.25x((4x39.1)+(2x16.0))=47.1g
Original post by muffin_selight
Sorry but I don't understand why you are using oxygen? I am still confused. This is what I got for this question:

7.8/(39.1x4)=0.5 moles
0.5/2 (because of the 2:1 ratio)=0.25
0.25x((4x39.1)+(2x16.0))=47.1g


To start, use n = m/Mr. You are given the mass of Potassium involved in the reaction (7.8) and so divide this by the molar mass of Potassium. This should give:

7.8 / 39 = 0.2 moles

You then have a 4:1 ratio and so dive the above answer by four:

0.2/4 = 0.05 moles

You then have a 1:2 ratio and so multiply by two:

0.1 moles of K2O

Use m = n x Mr, and so work out the molar mass for K20 and then multiply by the moles already calculated.

39x2 = 78

78 + 16 = 94g

94 x 0.1 = 9.4g

You didn't need to ratio to oxygen, but rather could have gone straight to K2O - I just wanted to make it clear and show full working.

You only mistake really is dividing by four lots of Potassium at the beginning - this isn't correct as you know the mass of Potassium and how much one mole weights, and so that is how you get moles.
Original post by Tullia
To start, use n = m/Mr. You are given the mass of Potassium involved in the reaction (7.8) and so divide this by the molar mass of Potassium. This should give:

7.8 / 39 = 0.2 moles

You then have a 4:1 ratio and so dive the above answer by four:

0.2/4 = 0.05 moles

You then have a 1:2 ratio and so multiply by two:

0.1 moles of K2O

Use m = n x Mr, and so work out the molar mass for K20 and then multiply by the moles already calculated.

39x2 = 78

78 + 16 = 94g

94 x 0.1 = 9.4g

You didn't need to ratio to oxygen, but rather could have gone straight to K2O - I just wanted to make it clear and show full working.

You only mistake really is dividing by four lots of Potassium at the beginning - this isn't correct as you know the mass of Potassium and how much one mole weights, and so that is how you get moles.


Thank you. I still don't really get why you don't divide by 4 at the beginning?

I realised I made a mistake in my working out when I done 7.8/(39.1x4)=0.5, it was actually 0.05. We both got 0.05 moles but I divided by 4 at the beginning; you didn't divide by 4 at the beginning but after you found out the number moles and the ratio, so would my way still be correct? I then done 0.05x(156.4+32 (which is the molar mass of 2K2O))=9.42g How did you get to:

39x2 = 78

78 + 16 = 94g

94 x 0.1 = 9.4g?

Sorry if I am being a pain :s-smilie:
Original post by muffin_selight
Thank you. I still don't really get why you don't divide by 4 at the beginning?

I realised I made a mistake in my working out when I done 7.8/(39.1x4)=0.5, it was actually 0.05. We both got 0.05 moles but I divided by 4 at the beginning; you didn't divide by 4 at the beginning but after you found out the number moles and the ratio, so would my way still be correct? I then done 0.05x(156.4+32 (which is the molar mass of 2K2O))=9.42g How did you get to:

39x2 = 78

78 + 16 = 94g

94 x 0.1 = 9.4g?

Sorry if I am being a pain :s-smilie:


I would suggest doing it in the way that I did, but that is because I think it will reduce the likelihood of you making a mistake (especially in more complex calculations). However, please do work in the way that you feel works best for you, as you wouldn't be penalised at all for the way you've done it.

I didn't bother to fully check your working and so assumed that was where you had made the mistake; apologies for that.

That is the molar mass for K2O. I've not been as precise as you because I don't have a Periodic Table at hand or anything. It's two lots of Potassium and one of Oxygen.
Original post by Tullia
I would suggest doing it in the way that I did, but that is because I think it will reduce the likelihood of you making a mistake (especially in more complex calculations). However, please do work in the way that you feel works best for you, as you wouldn't be penalised at all for the way you've done it.

I didn't bother to fully check your working and so assumed that was where you had made the mistake; apologies for that.

That is the molar mass for K2O. I've not been as precise as you because I don't have a Periodic Table at hand or anything. It's two lots of Potassium and one of Oxygen.


Okay, I think I am slowly begining to understand now. Why did you only do 1 lot of K2O when the equation states 2K2O?
I've done a couple of more questions, can you please check whether they are correct?

What mass of aluminium oxide is produced when 135g of aluminium is burned in oxygen?
2Al+3O2->Al2O3

2 moles of aluminium and 1 mole of aluminium oxide, therefore the ratio is 2:1
135/54=2.5
2.5x102=255g
However, shouldn't you divide by 2.5 by 2 as the ratio is 2:1?

What mass of iodine is produced when 7.1g of chlorine reacts with excess potassium iodide?
Cl2+2Kl->2KCl+I2
1 mole of chlorine and 1 mole of iodide, therefore the ratio is 1:1
7.1/71=0.1moles
0.1x(126.9x2)=25.38g

What mass of oxygen is formed when 735g of potassium chlorate decomposes?
2KClO3->2KCl+3O2
2 moles of potassium chlorate and 3 moles of oxygen, therefore the ratio is 2:3
735/2x(39.1+35.5+48)=3moles
(3/2)x3=4.5 (because of the ratio)
4.5x96=432g

What mass of hydrogen is produced when 195mg of potassium is added to water?
2K+2H2O->2KOH+H2
2 moles of potassium and 1 mole of hydrogen, therefore the ratio is 2:1
195/(2x39.1)=2.5moles
2.5/2=1.25moles (because of the ratio)
1.25x2=2.5g
I only just noticed that it is mg and not g so would you then divide by 1000 to get mg?

Thanks
Original post by muffin_selight
Okay, I think I am slowly begining to understand now. Why did you only do 1 lot of K2O when the equation states 2K2O?


Apologies for not explaining this very well.

In my calculation I worked out that I had 0.2 moles of Potassium. I then used the ratio of the equation to work out that I had 0.1 moles of Potassium Oxide. Whereas, you worked out that you had 0.05 moles.

You should be able to see that using the ratio means you can always deal with one lot of the molar mass. That is why is makes it easier. It effectively removes the number in front as you've dealt with that number when using the ratio of the moles.

What mass of aluminium oxide is produced when 135g of aluminium is burned in oxygen?
2Al+3O2->Al2O3

2 moles of aluminium and 1 mole of aluminium oxide, therefore the ratio is 2:1
135/54=2.5
2.5x102=255g
However, shouldn't you divide by 2.5 by 2 as the ratio is 2:1?


Again, this is the same problem as I've tried to explain above. You've chosen to divide by two lots of Aluminium at the start, and so you've effectively removed the ratio as the value is two times smaller than dividing by one lot of Aluminium (which is what I would suggest doing to make it simpler!).

What mass of iodine is produced when 7.1g of chlorine reacts with excess potassium iodide?
Cl2+2Kl->2KCl+I2
1 mole of chlorine and 1 mole of iodide, therefore the ratio is 1:1
7.1/71=0.1moles
0.1x(126.9x2)=25.38g


That is correct.

What mass of oxygen is formed when 735g of potassium chlorate decomposes?
2KClO3->2KCl+3O2
2 moles of potassium chlorate and 3 moles of oxygen, therefore the ratio is 2:3
735/2x(39.1+35.5+48)=3moles
(3/2)x3=4.5 (because of the ratio)
4.5x96=432g


You should get 288 grams as the answer.

You've been told you've got 735 grams of Potassium Chloride, and so you divide the mass by the molar mass (not twice the molar mass!). And then you have the 2:3 ratio and so work that out. Then you multiply by the mass of O2 (not three lots of O2, as you've already used the ratio the effectively remove the three!).

What mass of hydrogen is produced when 195mg of potassium is added to water?
2K+2H2O->2KOH+H2
2 moles of potassium and 1 mole of hydrogen, therefore the ratio is 2:1
195/(2x39.1)=2.5moles
2.5/2=1.25moles (because of the ratio)
1.25x2=2.5g
I only just noticed that it is mg and not g so would you then divide by 1000 to get mg?


You should get 0.005 grams. You wouldn't give an answer in milligrams unless asked, and so the milligrams just needs converting at the start.
I have done some more questions but these are about what mass is needed, not what mass is formed. I think I have done them correct, can you check whether my answers are okay please? I'd appreciate it very much.

What mass of oxygen is needed to react with 8.5g of hydrogen sulphide?
2H2S+3O2->2SO2+2H2O
2 moles of hydrogen sulphide and 3 moles of oxygen, therefore the ratio is 2:3
8.5/34=0.25moles
(0.25/2)x3=0.375 (because of the ratio)
0.375x32=12g

What mass of hydrogen is needed to react with 32g of copper oxide?
CuO+H2->Cu+H2O
1 mole of copper oxide and 1 mole of hydrogen, therefore there is a 1:1 ratio
32/(63.5+16.0)=0.4moles
0.4x2=0.8g

What mass of oxygen reacts with 192g of magnesium?
2Mg+O2->2MgO
2 moles of magnesium and 1 mole of oxygen, therefore ratio is 2:1
192/(2x24.3)=3.9moles
3.9x32=124.8g
Do I still have to divide 3.9 by 2 because of the ratio?

What mass of carbon monoxide is needed to react with 1kg of iron oxide?
Fe2O3+3CO->2Fe+3CO2
1 mole of iron oxide and 3 moles of oxygen, therefore the ratio is 1:3
1000/((2x55.8)+(48))=6.27moles
6.27x(3x(12+16))=526.68g

What mass of oxygen is needed to react with 184g of sodium?
4Na+O2->2Na2O
4 moles of sodium and 1 mole of oxygen, therefore the ratio is 4:1
184/(4x23)=2
2/4=0.5
0.5x32=16g

What mass of oxygen is needed to react with 4.5g of ethane?
2C2H6+7O2->4CO2+6H2O
2 moles of C2H6 and 7 moles of oxygen, therefore the ratio is 2:7
4.5/2x((12x2)+(2x6))=0.0625moles
(0.0625/2)x7=0.21875
0.21875x32=7g

Please can you check that my working is correct and if not, tell me where I am going wrong.

Thanks in advance.
Thank you.

So for the aluminium oxide question, is 255g the right answer? My working out would be:
135/27=5moles
5/2=2.5moles (because of the ratio)
2.5x102=255g

I understand the potassium chlorate question, my working out is as follows:
735/(39.1+35.5+48)=6moles
(6/2)x3=9 (because of the ratio)
9x32=288g

However, I still don't understand the last question. I tried working out the question a different way but got:
195mg=0.195g
0.195/39.1=0.005moles
(0.005/2)x2=0.005
0.005x2=0.01g
Original post by muffin_selight
I have done some more questions but these are about what mass is needed, not what mass is formed. I think I have done them correct, can you check whether my answers are okay please? I'd appreciate it very much.

What mass of oxygen is needed to react with 8.5g of hydrogen sulphide?
2H2S+3O2->2SO2+2H2O
2 moles of hydrogen sulphide and 3 moles of oxygen, therefore the ratio is 2:3
8.5/34=0.25moles
(0.25/2)x3=0.375 (because of the ratio)
0.375x32=12g

What mass of hydrogen is needed to react with 32g of copper oxide?
CuO+H2->Cu+H2O
1 mole of copper oxide and 1 mole of hydrogen, therefore there is a 1:1 ratio
32/(63.5+16.0)=0.4moles
0.4x2=0.8g

What mass of oxygen reacts with 192g of magnesium?
2Mg+O2->2MgO
2 moles of magnesium and 1 mole of oxygen, therefore ratio is 2:1
192/(2x24.3)=3.9moles
3.9x32=124.8g
Do I still have to divide 3.9 by 2 because of the ratio?

What mass of carbon monoxide is needed to react with 1kg of iron oxide?
Fe2O3+3CO->2Fe+3CO2
1 mole of iron oxide and 3 moles of oxygen, therefore the ratio is 1:3
1000/((2x55.8)+(48))=6.27moles
6.27x(3x(12+16))=526.68g

What mass of oxygen is needed to react with 184g of sodium?
4Na+O2->2Na2O
4 moles of sodium and 1 mole of oxygen, therefore the ratio is 4:1
184/(4x23)=2
2/4=0.5
0.5x32=16g

What mass of oxygen is needed to react with 4.5g of ethane?
2C2H6+7O2->4CO2+6H2O
2 moles of C2H6 and 7 moles of oxygen, therefore the ratio is 2:7
4.5/2x((12x2)+(2x6))=0.0625moles
(0.0625/2)x7=0.21875
0.21875x32=7g

Please can you check that my working is correct and if not, tell me where I am going wrong.

Thanks in advance.


What mass of oxygen is needed to react with 8.5g of hydrogen sulphide?
2H2S+3O2->2SO2+2H2O
2 moles of hydrogen sulphide and 3 moles of oxygen, therefore the ratio is 2:3
8.5/34=0.25moles
(0.25/2)x3=0.375 (because of the ratio)
0.375x32=12g


Correct.

What mass of hydrogen is needed to react with 32g of copper oxide?
CuO+H2->Cu+H2O
1 mole of copper oxide and 1 mole of hydrogen, therefore there is a 1:1 ratio
32/(63.5+16.0)=0.4moles
0.4x2=0.8g


Correct.

What mass of oxygen reacts with 192g of magnesium?
2Mg+O2->2MgO
2 moles of magnesium and 1 mole of oxygen, therefore ratio is 2:1
192/(2x24.3)=3.9moles
3.9x32=124.8g
Do I still have to divide 3.9 by 2 because of the ratio?


I got 126.4, but that is because it is 3.95 and not 3.9. Otherwise, correct. And no, you don’t have to divide the 3.9 by two as you’ve removed the two by dividing by two lots of Magnesium at the beginning. Whereas, I divided by two because I did the following:

192/24.3 = 7.9

Ratio is 2:1, so 7.9 / 2 = 3.95

3.95 x 32 = 126.4

You divided by two at the beginning and so got 3.95 straight away. However, I would advise doing the calculations in the way I do them to avoid any confusion. My method effectively removes the numbers before you do the last bit.

What mass of carbon monoxide is needed to react with 1kg of iron oxide?
Fe2O3+3CO->2Fe+3CO2
1 mole of iron oxide and 3 moles of oxygen, therefore the ratio is 1:3
1000/((2x55.8)+(48))=6.27moles
6.27x(3x(12+16))=526.68g


Correct.

What mass of oxygen is needed to react with 184g of sodium?
4Na+O2->2Na2O
4 moles of sodium and 1 mole of oxygen, therefore the ratio is 4:1
184/(4x23)=2
2/4=0.5
0.5x32=16g


Incorrect.

You have the mass of Sodium, and can look up the molar mass. So 184/23 = 8. Use the 4:1 ratio to give 2. Multiply by 32 to give 64.

What mass of oxygen is needed to react with 4.5g of ethane?
2C2H6+7O2->4CO2+6H2O
2 moles of C2H6 and 7 moles of oxygen, therefore the ratio is 2:7
4.5/2x((12x2)+(2x6))=0.0625moles
(0.0625/2)x7=0.21875
0.21875x32=7g


Incorrect.

You have the mass of Ethane, and you can work out the molar mass. So 4.5/30 = 0.15. Use the ratio 2:7 to give 0.525. Multiply by 32 to give 16.8.
Original post by muffin_selight
Thank you.

So for the aluminium oxide question, is 255g the right answer? My working out would be:
135/27=5moles
5/2=2.5moles (because of the ratio)
2.5x102=255g

I understand the potassium chlorate question, my working out is as follows:
735/(39.1+35.5+48)=6moles
(6/2)x3=9 (because of the ratio)
9x32=288g

However, I still don't understand the last question. I tried working out the question a different way but got:
195mg=0.195g
0.195/39.1=0.005moles
(0.005/2)x2=0.005
0.005x2=0.01g


Yeah, the first two answers are correct.

The last one was this:

What mass of hydrogen is produced when 195mg of potassium is added to water?


195 milligrams = 0.195 grams

0.195/39.1 = 0.005 moles of Potassium

Ratio is 2:1, so 0.0025 moles of Hydrogen

m = n x Mr, so m = 0.0025 x 2 = 0.005 grams of Hydrogen

You've made the same mistake of multiplying by two despite having dealt with the two earlier on when doing the ratio. Remove the last step in your calculation and you're correct. I would advise you to break it up into the individual steps rather than doing both steps at once. So much easier.
Original post by Tullia
You have the mass of Ethane, and you can work out the molar mass. So 4.5/30 = 0.15. Use the ratio 2:7 to give 0.525. Multiply by 32 to give 16.8.


I don't understand where you got the 30 from, other than that, I think I am beginning to understand now. :smile:
Original post by muffin_selight
I don't understand where you got the 30 from, other than that, I think I am beginning to understand now. :smile:


Ethane is C2H6. This is (12 x 2) + (6) = 30. The molar mass of Ethane is 30 g/mol.
Original post by Tullia
Ethane is C2H6. This is (12 x 2) + (6) = 30. The molar mass of Ethane is 30 g/mol.


Okay, thanks :smile:
Reply 19
Original post by muffin_selight
Hello,

I did this in school. So the equation is already balanced.
Forst you are going to find the atomic mass for each formula. I got 4K=156,, O2=32,, 2K2O=188.
Next you need to get the 156 to 1, so that you can make it 7.8. Do what I did was 156/156=1 1*7.8=7.8.
Now you need to do what you did to this, to the other formulas.
So, 32/156, Answer*7.8
188/156, Answer*7.8
From this I got:
4k=7.8g
O2=116g
2k2O=9.4
So, the answer is 9.4
Hope this helps!

Quick Reply

Latest