a.How many grams of HCL are required to react completely with 4.30 grams of zinc?
b. How many molecules of gas will be produced?

Hello teatea,

a. 2HCl + Zn -> ZnCl2 + H2
We have 4.30 grams of Zinc.  Since there are 65.37 grams of Zinc in
one mole (found using any periodic table), we perform 4.30/65.37 to
find how many moles of Zinc there are, which comes out to 0.06578 mol.
 By looking at the equation, we see that there are 2 mol of HCl for
every 1 mol of Zn.  This means that there are 2*0.06578 mol of HCl, or
0.1316 mol.  To convert this to grams, we must multiply it by the
molar mass of HCl, which is 33.45 g (found by adding the molar mass of
H and Cl).  The calculation is 0.1316*33.45 = 4.40 grams of HCl.

b. Since H2 and Zn both are created in equal proportions (both have 1
in front of them), the molecules of gas of each will be the same. 
Since we know there are 0.06578 mol of Zn from part (a), we must
simply multiply this by Avagadro's number, 6.02*10^23 to find the
amount of molecules of gas that will be produced.  6.02*10^23*0.06578
= 3.96*10^22 molecules of gas will be produced.

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rating it.

Thanks,
chis-ga