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Solving mole calculations with equations

Magnesium and hydrochloric acid react to form magnesium chloride and hydrogen gas. Calculate the volume of H2 (g) produced at rtp when 4.8g Mg reacts with HCl. (Mg=24)
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1 Answer

First, Karishma, you have to write out the balanced equation.
 
Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
 
So 1 mole of Mg will produce 1 mole of H2.  Since Mg has a molar mass of 24g,
 
4.8g x (1 mole/24g) = 0.20 mole Mg
 
So 0.20 mole of H2 are produced.  At STP, 1 mole of any gas occupies 22.4 liters, so
 
0.20 mole x (22.4 liters/1 mole)
 
And you produce 4.48 liters of H2. (4.5 liters using significant digits)