Question

What mass of zinc would need to react with hydrochloric acid in order to produce 25.0 mL of hydrogen gas collected over water at 25 degrees celsius and 105 kPa.
How do you solve this using the combined gas laws?
The answer is 0.067 grams but how do you get there?

Answer 1

1) From a Table of Water Vapour Pressure (in kPa), find for water at 25 C, pressure = 3.17 kPa.
105.3 kPa - 3.17 kPa = 101.83 kPa

2) gas law constant, R = 0.0821 L-atm/mol-K
Convert other units to match those used in R:
101.83 kPa/101.325 kPa/atm = 1.005 atm
25 mL = 0.025 L
25 C + 273.15 = 298.15 K

Moles of H2:
Using PV = nRT, substitute the above and solve for n to get 0.001026 mol H2.

Now balance the chemical equation:
Zn + 2HCl --> H2 + ZnCl2
Ratio of Zn to H2 is 1:1, therefore,
0.001026 mol H2 x 1 mol Zn/1mol H2
=0.001026 mol Zn

0.001026 mol Zn x 65.38 g/mol Zn
= 0.067 g Zn

Answer 2

I know how to solve it using the ideal gas law, but how do you solve it using the combined gas laws?

Answer 3

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Gas Law Problems- Combined Gas Law

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