Calculate the percent purity of a sample of zinc metal if 325 mL of hydrogen gas

Question

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 220.0 degrees Celsius and an atmospheric pressure of 745.0 mm Hg when 1.12 g of the sample is reacted with excess HCl: Zn(s)+2HCl(aq)->H2(g)+ZnCl2(aq). The vapor pressure of water at 22.0 degrees Celsius is 19.8 mm Hg. Assume that any impurities in the zinc metal do not react with HCl. 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 220.0 degrees Celsius and an atmospheric pressure of 745.0 mm Hg when 1.12 g of the sample is reacted with excess HCl: Zn(s)+2HCl(aq)->H2(g)+ZnCl2(aq). The vapor pressure of water at 22.0 degrees Celsius is 19.8 mm Hg. Assume that any impurities in the zinc metal do not react with HCl.

Explanation / Answer

Pressure due to Hydrogen gas= 745-19.8

=725.2 mm of Hg

now using the ideal gas equation,

PV=nRT,

(725.2/760)*0.325=n*0.0821*(220+273)

or n=0.0076 moles

so actual mass of zinc present = 0.0076*65

=0.494 grams

so percent purity = 0.494*100/1.12

=44.1 %

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