Chemistry, can i check an answer?

Question

1.0g of zinc was placed in 20cm^3 of 2 mol l^-1 hydrochloric acid. After 20 seconds the zinc was removed, washed, dried and reweighed. The remaining zinc weighed 0.35g. Calculate the number of moles of hydrochloric acid used up in the 20s.

I got 0.01moles. My friend got 0.014moles. What is the actual answer? and if we are both wrong, how do you do it? (I should know this considering i just got a 1,1,1 in SG. but its all gone through the hollidays)

Answer 1

havent u just got the same answer but ones rounded.....

Answer 2

Well, your answer has the right # of significant figures.  Here's the method I would use:

2HCl(aq) + Zn(s) --> ZnCl2(aq) + H2(g)

The amount of zinc eaten away was 1.0g - 0.35g = 0.65g.

The molar mass of zinc is 65.39 g/mol, so that would be .65/65.39 or .0099403579 moles of zinc.  Since it takes 2 mol HCl to dissolve each mole of zinc, the number of moles used would be 0.0198807157 or 0.02 moles.  

I can't be sure, but I think that for this problem, you don't even have to know all the other information.