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Whenever we are looking at a reaction, we always want to make sure it it is balanced before we start doing any calculations. Since this one is balanced, we are ready to start!
When we look at the coefficients of the balanced chemical equation, they tell us about the molar ratios of the substances. 1 mole of Zn reacts with 2 moles of acid to form 1 mole of ZnCl2 and 1 mol of H2.
In this problem, we are given the amount of Zn so we can assume that we have sufficient amount of acid to react with all of the Zn. Knowing the mass of Zn will allow me to use the molar mass to determine the moles of Zn and then I can use the molar ratio to find the moles of H2. We'll set that up first and then go back and look at how to get from moles of H2 to liters of H2.
98.1 g Zn (1 mol / 65.4 g) (1 mol H2 / 1 mol Zn) = 1.5 mol H2
Now, we know how many moles of Zn we have, but we need to convert the moles of H2 to liters of H2 using the ideal gas law. The clue to know that we need to do this is that we are given the temperature and pressure conditions (STP = 1 atm pressure and 0 degC (273 K))
The ideal gas equation PV = nRT will be used to calculate the volume of gas.
(1 atm)V = (1.5 mol)(0.08206 L atm/mol K)(273 K)
V = 33.6 L H2
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