Question

How many grams of water can be formed when $4.0$ grams of hydrogen are ignited with $4.0$ grams of oxygen?

• A. $0.50$
• B. $2.5$
• C. $4.5$
• D. $8.0$
• E. $36$

Answer 1

The correct option is C $4.5$

$2H_2+O_2\to 2H_{2}O$
The molar masses of hydrogen, oxygen and water are 2 g/mol, 32 g/mol and 18 g/mol respectively.
The number of moles is the ratio of mass to molar mass.
The number of moles of hydrogen $=\frac{4.0g}{2g/mol}=2.0mol$
The number of moles of oxygen $=\frac{4.0g}{32g/mol}=0.125$ moles
0.125 moles of oxygen will react with $0.125\times 2=0.250$ moles of hydrogen.
However, 2.0 moles of hydrogen are present.
Hence, hydrogen is present in excess and oxygen is the limiting reagent.
0.125 moles of oxygen will give $2\times 0.125=0.250$ moles of water.
Mass of water formed $=0.250mol\times 18g/mol=4.5g$