Question

Sodium reacts with water to produce hydrogen gas and sodium hydroxide. How many grams of H_2(g) are produced when 5.0 g Na reacts with 100.0 g of water? 

Answer 1

$2Na+2H_2O \rightarrow 2NaOH + H_2$

M(Na)=23 g/mol

n(Na)=5/23=0.217 mol

$M(H_2O)=1x2+16=18 g/mol$

$n(H_2O)=100/18=5.556 mol$

As we can see from reaction, $[Na]:[H_2O]=1:1$

So Na is limited reagent. From reaction we know that $n(H_2)=n(Na)/2=0.109 mol$

$M(H_2)=2x1=2 g/mol$

$m(H_2)=M(H_2)xn(H_2)=2x0.109=0.217 g$