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

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

No, I have an OCR one, it doesn't explain the calculations very well

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

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

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?

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?

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

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

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

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

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.

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