Examine the following unbalanced equation.

HCl (g) + O2 Cl2 (g) + H2O (g)

Assuming that all the gases in this reaction are at STP, what number of molecules of hydrogen chloride gas are needed to completely react with oxygen to yield 50.00 mL of chlorine gas?

1) Determine the number of moles of chlorine gas

At STP 1 mol = 22.4 liter

=> [1mol/22.4 liter] * 0.050 liter = 0.00223 mol Cl2 (g)

2) Write the balanced equation

4HCl (g) + O2 - - > 2Cl2 (g) + 2H2O (g)

3) Write the ratios of the compound on which you are focused.

4 HCl : 2 mol Cl2

Then, 0.00223 mol Cl2 * [4mol HCl / 2 mol Cl2] = 0.00446 mol HCl

5) Use Avogadro's number to find the number of molecules:

0.00446 mol * 6.02*10^23 = 2.68 * 10^21 molecules of HCl

Answer: 2.68 * 10^21 molecules