Question

$30$ g of magnesium and $30$ g of oxygen are allowed to react. The residual mixture contains:

• A. $45$ g of magnesium oxide and $15$ g of oxygen
• B. $50$ g of magnesium oxide and $10$ g of oxygen
• C. $60$ g of magnesium oxide only
• D. $45$ g of magnesium oxide and $20$ g of oxygen

Answer 1

Answer: (B)

$50$ g of magnesium oxide and $10$ g of oxygen

The reaction is $2Mg+O_2\to 2MgO$.

Thus, $2$ moles of magnesium react with $1$ mole of oxygen to form $2$ moles of magnesium oxide.

The molecular weights of magnesium, oxygen and magnesium oxide are $24,32$ and $40$ g respectively.

$30$ g of magnesium and $30$ g of oxygen corresponds to $1.25$ and $0.9375$ moles respectively.

$1.25$ moles of magnesium will react with $0.625$ moles of oxygen to produce $1.25$ moles of magnesium oxide.

Thus, $30$ g of magnesium will react with $20$ g of oxygen to produce $50$ g of magnesium oxide.

Thus, the resulting mixture contains $50$ g of magnesium oxide and $30-20=10$ g of oxygen.