Need Help with moles urgently! :(

i) Just use the formula Moles = Mass/ Molar Mass
(having found the value of the Mr of the MgSO4 by adding together the Ar of all the elements in their proportions)
ii) Same as above
iii) You've now got the two values for the numbers of moles of each in the substance, which is the first step in finding the empirical formula. The next is to find the simplest ratio between the two: if it's not obvious, then just divide both by the smallest value. Given the phrasing of the question, that'll then give you a ratio of moles of MgSO4: moles of H20 in the form 1:x, with x being the value you're trying to find.

Reply 3

OP

Thanks guys, now it looks so easy. Ha! I'll remember that for tomorrow.

I have another question, sorry, should have put it in first post I guess:

Calcium carbonate, CaCO3, reacts with hydrochloric acid.

7.50 x 10^3 mol CaCO3 reacts with 0.200 mol dm-3 HCl

i) Calculate the volume, in cm^3, of 0.200 mol dm-3 HCl required to react with 7.50 x 10^3 mol CaCO3.

ii) Calculate the volume, in cm^3, of CO2 formed at room temperate and pressure.

I know it's probably basic stuff but like I said I have struggled with it this term. Thanks again. :smile:

Reply 4

What paper is that CaCO3 question from cuz I don't get why it would be 10^3 .. it's a bit much isn't it? Ironically I did a very similar question just now!

Reply 5

SpottySocks

Thanks guys, now it looks so easy. Ha! I'll remember that for tomorrow.

I have another question, sorry, should have put it in first post I guess:

I know it's probably basic stuff but like I said I have struggled with it this term. Thanks again. :smile:

i) Firstly, you'll need a balanced equation, which will show that a mole of Calcium Carbonate reacts with 2 moles of HCL to give a mole of CaCl2, a mole of CO2, and a mole of water.
So, if there's 7500 moles of Calcium Carbonate, then that means there'll be twice the number of moles of HCl, so 15,000 moles of that. Then use the equation Volume = Moles/ Concentration to find the volume of HCl (remembering to convert from decimetres into cm3 after you've done that)
i) The ratio is 1:1 of Calcium Carbonate forming Carbon Dioxide, so 7500 moles of the former will form 7500 moles of the latter. One mole is 24dm^3 (=24,000 cm^3) at room temperature and pressure, so just multiply the number of moles by this molar volume, to give you the total volume.

Reply 6

TwentyNine

What paper is that CaCO3 question from cuz I don't get why it would be 10^3 .. it's a bit much isn't it? Ironically I did a very similar question just now!

I don't get what you mean; there's no reason for it being that value, it's just one they've randomly chosen to use in the question.

Reply 7

Revolution is my Name

I don't get what you mean; there's no reason for it being that value, it's just one they've randomly chosen to use in the question.

Haha yeah, but the question I just did, it was more like 10^-3 so I was getting confused

Reply 8

TwentyNine

Haha yeah, but the question I just did, it was more like 10^-3 so I was getting confused

Yeah, you're right, the values are usually quite small. Them being bigger is probably a good thing though, as I suppose you're considering them as actual numbers and you'll be more likely to notice if something goes amiss, whereas if they're small numbers in standard form there's more bunging them into the calculator and blindly following what it says.

Reply 9

Revolution is my Name

Yeah, you're right, the values are usually quite small. Them being bigger is probably a good thing though, as I suppose you're considering them as actual numbers and you'll be more likely to notice if something goes amiss, whereas if they're small numbers in standard form there's more bunging them into the calculator and blindly following what it says.

Yep there is :]

Reply 10

OP

For part 1 I got 3cm^3, that can't be right? Do I multiply the 0.200 by 1000? The number would be too big then wouldn't it?

I did (15,000/0.200)/1,000

Gawd I sound so dumb lol

and yep sorry missed out the -! It was 10^-3.

Reply 11

SpottySocks

For part 1 I got 3cm^3, that can't be right? Do I multiply the 0.200 by 1000? The number would be too big then wouldn't it?

I did (15,000/0.200)/1,000

Gawd I sound so dumb lol

and yep sorry missed out the -! It was 10^-3.

15,000 moles and a concentration of 0.2 mol dm^-3 gives:
Volume = Moles/ Concentration
= 15,000/ 0.2
= 75,000 dm^3
1 dm^3= 1,000 cm^3, hence to convert from dm^3 to cm^3 we need to multiply by 1,000, giving:
75,000,000 cm^3

By the looks of your answer, I think you accidentally multiplied Moles and concentration, instead of dividing the first by the second.

Reply 12

OP

Revolution is my Name

15,000 moles and a concentration of 0.2 mol dm^-3 gives:
Volume = Moles/ Concentration
= 15,000/ 0.2
= 75,000 dm^3
1 dm^3= 1,000 cm^3, hence to convert from dm^3 to cm^3 we need to multiply by 1,000, giving:
75,000,000 cm^3

By the looks of your answer, I think you accidentally multiplied Moles and concentration, instead of dividing the first by the second.

Ahh I see thank you. I got that number first but thought it didn't look right. Not much looks right when it comes to me and Chemistry. :rolleyes:

Thanks for your help I am understanding it better now. :smile:

Reply 13

SpottySocks

For part 1 I got 3cm^3, that can't be right? Do I multiply the 0.200 by 1000? The number would be too big then wouldn't it?

I did (15,000/0.200)/1,000

Gawd I sound so dumb lol

and yep sorry missed out the -! It was 10^-3.

Don't call yourself dumb!

Okay moles=concentration X volume right? So if you want to know the volume you simply re-arrange your formula.. hence volume = moles/concentration
So thats: 7.50 x 10^-3 / 0.200 = 0.0375 dm^3 if you want, multiply that by 1000 to get cm^3 which will = 37.5 cm^3

Reply 14

TwentyNine

Don't call yourself dumb!

Okay moles=concentration X volume right? So if you want to know the volume you simply re-arrange your formula.. hence volume = moles/concentration
So thats: 7.50 x 10^-3 / 0.200 = 0.0375 dm^3 if you want, multiply that by 1000 to get cm^3 which will = 37.5 cm^3

There's twice as many moles of HCl as there is of Calcium Carbonate, so that should be 1.5x10^-2

Reply 15

Revolution is my Name

There's twice as many moles of HCl as there is of Calcium Carbonate, so that should be 1.5x10^-2

Ahh yeahh I forgot the equation, well, I was hoping to outline the basic idea =]

Reply 16