11. Small quantities of hydrogen gas can be prepared in the laboratory by the ad
ID: 556339 • Letter: 1
Question
11. Small quantities of hydrogen gas can be prepared in the laboratory by the addition of aqueous hydrochloric acid to metallic zinc. Provide the balanced chemical equation: Typically, the hydrogen gas is bubbled through water for collection and becomes saturated with water vapor. We must take the presence of water vapor into account when calculating the properties of the gas sample. Suppose 240. mL of hydrogen gas is collected at 30.0°C and has a total pressure of 1.032 atm by this process. What is the partial pressure of hydrogen gas in the sample? How many moles of hydrogen gas are present in the sample? How many grams of zinc must have reacted to produce this quantity of hydrogen? (The vapor pressure of water is 32 torr at 30.00)Explanation / Answer
Zn(s) + 2 HCl(aq) ZnCl2(aq) + H2(g)
Given that the vapor pressure of water is 32 torr at 30.0C.
Change the unit from torr to atm as follows:
32 torr x 1 atm/760 torr = 0.042 atm
Pressure due to H2 gas = total pressure – pressure of water vapor
= 1.031 atm - 0.042 atm
= 0.989 atm
T = 30 C = 303K
now calculate the mole of H2 as follow:
PV = nRT
n = PV/RT
n = (0.989 atm)(0.240 L)/(0.082 L-atm/deg-mol)(303 deg)
n = 0.00955 mole of H2 gas produced
Since it takes 1 mole Zn to produce 1 moles H2 gas, the amount of Zn needed is also 0.00955 moles
grams of Zn needed = 0.00955 moles x 65.4 g/mol = 0.624 grams
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