Question

1.0 g of magnesium is burnt with 0.56 g O2 in a closed vessel. Which reactant is left in excess and how much? (At. wt. Mg = 24 ; O = 16)

• A. Mg, 0.16 g
• B. O2, 0.16 g
• C. Mg, 0.44 g
• D. O2, 0.28 g

Answer 1

Answer: (A)

Mg, 0.16 g

Answer 2

The balanced chemical equation is $\underset{\begin{smallmatrix} \\\ \\\ 24g \end{smallmatrix}}{\mathop{Mg}}\,+\underset{\begin{smallmatrix} \\\ 16g \end{smallmatrix}}{\mathop{\frac{1}{2}{{O}_{2}}}}\,\xrightarrow{\,}\underset{\begin{smallmatrix} \\\ \\\ 40g \end{smallmatrix}}{\mathop{MgO}}\,$
From the above equation, we note that 24 g Mg reacts with 16 g O2
Therefore, 1.0 g Mg reacts with $\frac{16}{24}\times 0.67g\,{{O}_{2}}=0.67g\,{{O}_{2}}.$
However, only 0.56 g of O2 is available.
Thus, O2 is the limiting reagent.
16 g O2 then reacts with 24 g Mg.
$\therefore$ 0.56 g O2 will react with Mg $=\frac{24}{16}\times 0.56=0.84g$
The amount of Mg left unreacted = 1.0 - 0.84g Mg = 0.16g Mg.
Hence, Mg is present in excess and 0.16 g Mg is left behind unreacted.

So, the correct option is (A): Mg, 0.16 g