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Aqa a level chemistry exam question help!!!

Zinc forms many different salts including zinc sulfate, zinc chloride and zinc fluoride.
(a) People who have a zinc deficiency can take hydrated zinc sulfate (ZnSO4.xH2O) as a dietary supplement.
A student heated 4.38 g of hydrated zinc sulfate and obtained 2.46 g of anhydrous zinc sulfate.
Use these data to calculate the value of the integer x in ZnSO4.xH2O
Show your working.

I know why how to get the ratio of 1:7, but why is the answer x=7? can someone help thanks

Reply 1

pashionfruit

The ratio 1:7 is the ratio of ZnSO4 to H2O. Therefore for every 1 mole of ZnSO4, you will have 7 moles of H2O.
As the question asks for the number of moles which joins to 1 mole of ZnSO4, x=7 :smile:

Reply 2

zak.820

how do u do this question?

Reply 3

frostedfaries

basically you have to find the mass of the water first so u have to do, mass of hydrated zinc sulfate - anhydrous zinc sulfate (4.38-2.46=1.92g). Now you have to put them into ratio, so ZnSO4 : H2O and you do the mass of each (2.46 and 1.92) and divide it by its Mr- (2.46g/161.5)(1.92g/18). Then to get a simplified ratio you have to find its empirical formula, so once you divide the mass by the Mr you find which one has the lowest number (this is ZnSO4) and divide those values by that smallest value. You will get 1:6.9589... but you can round it up and it will be 1:7. So the formula will be ZnSO4 7H2O