AP Chemistry

Small quantities of hydrogen gas can be prepared in the laboratory by the addition of aqueous hydrochloric acid to metallic zinc.

Zn(s)+2HCl(aq) -> ZnCl2(aq)+ H2(g)

Typically, the hydrogen gas is bubbled through water for collection and becomes saturated with water vapor. Suppose 240 mL of hydrogen gas is collected at 30*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 grams of zinc must have reacted to produce this quantity of hydrogen? (The vapor pressure of water is 32 torr at 30*C)

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  1. Total pressure = partial pressure H2O vapor + partial pressure H2 gas. You know two of the three, calculate the third one.

    Use PV = nRT to calculate n = moles of H2 gas, then use stoichiometry to determine grams of Zn. Here is an example of a stoichiometry problem I've posted. Just follow the steps after you find moles from the PV = nRT equation.
    http://www.jiskha.com/science/chemistry/stoichiometry.html

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  2. hydrogen = 0.990 atm, 9.55 x 10-3 mol H2, 0.625 g Zn

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