Question 14 Marks

In a balanced chemical reaction, equal number of atoms are present on both sides of reaction. A balanced chemical reaction is based on law of conservation of mass which means that total mass of reactants and products participating in a reaction must be equal. For example, a balanced chemical equation of burning of magnesium in oxygen to form magnesium oxide is written as:
$2 \mathrm{Mg}+\mathrm{O}_{2} \longrightarrow 2 \mathrm{MgO}$
The mass of reactants ( $2 \times 24+32=80$ ) is equal to the mass of products [ $2 \times(24+16)=80$ ].
i. In a reaction, 35 g of reactant, PQ breaks down into 20 g of product, P and an unknown amount of product, Q. Find the amount of product Q.
ii. The solid mercury (II) oxide is heated, and liquid mercury and oxygen gas are produced. Mention balanced chemical reaction.
iii. Which laws are satisfied by a balanced chemical equation?
OR
In the given chemical reaction,
$2 \mathrm{C}_{6} \mathrm{H}_{6(\mathrm{l})}+15 \mathrm{O}_{2(\mathrm{~g})} \longrightarrow \mathrm{mCO}_{2(\mathrm{~g})}+\mathrm{nH}_{2} \mathrm{O}_{(\mathrm{l})}$
Find the values of $m$ and $n$ respectively.

Answer

According to law of conservation of mass,
Mass of PQ = Mass of P + Mass of Q
$\therefore$ Mass of Q $=(35-20) \mathrm{g}=15 \mathrm{~g}$
ii. 2 moles of mercury (II) oxide produce 2 moles of mercury and one mole of oxygen gas.
$2 \mathrm{HgO}_{(\mathrm{s})} \longrightarrow 2 \mathrm{Hg}_{(\mathrm{l})}+\mathrm{O}_{2(\mathrm{~g})}$
iii. The law of conservation of mass is satisfied by a balanced chemical equation.
OR
12 and 6

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