When a 118-mL flask is evacuated, it has a mass of 97.129 g. Upon filling it with 768 torr of helium gas at 35 °C, the mass increases to 97.171 g. Based on this information, can we determine if the helium gas was pure?

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Calculate the mass of the helium gas by subtracting the mass of the evacuated flask from the mass of the flask with helium.

Convert the pressure from torr to atm using the conversion factor: 1 atm = 760 torr.

Convert the temperature from Celsius to Kelvin by adding 273.15 to the Celsius temperature.

Use the ideal gas law, PV = nRT, to calculate the number of moles of helium gas. Use the mass of helium, the molar mass of helium (4.00 g/mol), and the number of moles to find the molar mass.

Compare the calculated molar mass with the known molar mass of pure helium to determine if the gas is pure.

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Modern pennies are composed of zinc coated with copper. A student determines the mass of a penny to be 2.482 g and then makes several scratches in the copper coating (to expose the underlying zinc). The student puts the scratched penny in hydrochloric acid, where the following reaction occurs between the zinc and the HCl (the copper remains undissolved): Zn(s) + 2 HCl(aq)¡ H2( g) + ZnCl2(aq) The student collects the hydrogen produced over water at 25 °C. The collected gas occupies a volume of 0.899 L at a total pressure of 791 mmHg. Calculate the percent zinc (by mass) in the penny. (Assume that all the Zn in the penny dissolves.)

A 2.85-g sample of an unknown chlorofluorocarbon decomposes and produces 564 mL of chlorine gas at a pressure of 752 mmHg and a temperature of 298 K. What is the percent chlorine (by mass) in the unknown chlorofluorocarbon?

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