You are Here: Home

# Hard Chemistry AQA AS question

Announcements Posted on
Would YOU be put off a uni with a high crime rate? First 50 to have their say get a £5 Amazon voucher! 27-10-2016
1. Can anyone show me how to do this - this question was on the specimen paper:

After reaction of some zinc metal with excess sulfuric acid, a student collected 40.8 g of ZnSO4.7H2O crystals. The yield of crystals was 70.0%. What was the original mass of zinc used?

The answer is 13.3g - can someone show me how?
2. Hey man, is there an equation that accompanies the question? I think a similar Q came up last year CHEM 1.
3. Work out what a 100% yield would be - 58. 3
With this number divide by the Mr of ZnSO4. 7H2O then multiply by the Mr of Zn metal .
I think this is right
4. Mr: ZnS04.7H20 = 287.4 100% yield gives 58.3g so 58.3/287.4= 0.203 mol so there are 0.203 mol Zn to start therefore 0.203*65.4=13.3g
5. (Original post by Katie#1)
Work out what a 100% yield would be - 58. 3
With this number divide by the Mr of ZnSO4. 7H2O then multiply by the Mr of Zn metal .
I think this is right
why do you multiply by the Mr
6. guys why do you multiple by the Mr
7. (Original post by Enderbat1999)
guys why do you multiple by the Mr
Moles * Mr = Mass
8. (Original post by Smalltrims)
Mr: ZnS04.7H20 = 287.4 100% yield gives 58.3g so 58.3/287.4= 0.203 mol so there are 0.203 mol Zn to start therefore 0.203*65.4=13.3g
Why do we assume the mols are the same?

Zn + H2SO4 = ZnSO4.7H2O

This doesnt ballence, so wont mols be different and affect the answer?
9. 1 mole of zinc reacts to give you 1 mole of hydrated zinc sulfate, so the number of moles of zinc in the product will be the same as the number of moles of zinc in the reactant.
10. (Original post by VanEkerDr)
1 mole of zinc reacts to give you 1 mole of hydrated zinc sulfate, so the number of moles of zinc in the product will be the same as the number of moles of zinc in the reactant.
But there are more H and O's on the right of equation?
11. (Original post by 123chem)
But there are more H and O's on the right of equation?
This is because the solution is aqueous so you will have H and O in it. However, the number of mol of zinc cannot change from left to right. If you look at your product, you only have 1 mol of zinc.

## Register

Thanks for posting! You just need to create an account in order to submit the post
1. this can't be left blank
2. this can't be left blank
3. this can't be left blank

6 characters or longer with both numbers and letters is safer

4. this can't be left empty
1. Oops, you need to agree to our Ts&Cs to register

Updated: May 29, 2016
TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Today on TSR

Find out here

### How to cope with a heavy workload

Poll
Study resources

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.