Calculate the percent purity of a sample of zinc metal if 325 mL of hydrogen gas is collected by water displacement at a temperature of 22.0 C and an atmospheric pressure of 745.0 mm Hg when 1.12 g of the sample is reacted with excess HCl: Zn + 2 HCl = H2 + ZnCl2. The vapor pressure of water at 22.0 C is 19.8 mm Hg. Assume that any impurities in the zinc metal do not react with HCl.

Answer 1

Use PV = nRT and solve for n = number of moles at the conditions listed.
For p you must use ptotal = pH2 + pH2O
ptotal = 745 mm
pH2O = 19.8 mm from the problem. Solve for pH2 (which is dry H2), then remember to convert to atm. 760 mm = 1 atm.

When you have n, that's mols H2.
From the equation 1 mol H2 = 1 mol Zn
Then g Zn = mols Zn x atomic mass Zn.

%Zn = (g Zn/mass sample)*100 = ?