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    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?
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    Hey man, is there an equation that accompanies the question? I think a similar Q came up last year CHEM 1.
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    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
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    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
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    (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
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    guys why do you multiple by the Mr
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    (Original post by Enderbat1999)
    guys why do you multiple by the Mr
    Moles * Mr = Mass
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    (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?
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    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.
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    (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?
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    (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.
 
 
 
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