# Practice Questions on Reacting masses

Solve the following Practice Questions on Reacting masses using balanced Chemical equations

Q.1 20 g Magnesium ribbon is burnt in air with white light. Calculate the mass of Magnesium oxide (MgO) formed in this reaction.

Hint: Write the balanced chemical equation for the reaction between Mg and Oxygen gas.

2Mg(s) + O2 (g) → 2MgO(s)

Use the Molar mass of Mg and MgO according to balanced equation and calculate the mass of MgO.

Mr of Mg= 24 g/mol

Mr of O = 16 g/ mol

So MgO = 24+16 = 40 g/mol

2Mg(24×2) + O2 (g) → 2MgO(40×2)

So According to balanced equation,

48 g Mg forms MgO = 80 g

Hence 20 g Mg will form MgO = (20X80)/48

=33.33 g thus 20 g Mg will form 33.33 g MgO

We did not take Oxygen in to account because the Oxygen is not a limiting reactant and it is enough in the reaction.

Q2. 25 g Li2O(s) is reacted with 100 cm3 of  0.75mole/dm3  HCl solution as per below equation:

Li2O+ 2HCl(aq) → 2LiCl (aq) + H2O(l)

Calculate the amount of LiCl formed in the reaction.

Q3. 2.5 moles of MgO  reacts with  2.5 moles of HCl. Determine the limiting reactant and calculate the amount of  MgCl2 formed in the reaction.

MgO(s) + 2HCl (aq) → MgCl2(aq) + H2O(l)

Q.4  150g Phosphorus reacts with 200 g Oxygen gas , determine the limiting reactant and calculate the mass of Phosphorus oxide(P4O10 ) formed in this reaction.

Read `Mole concept`.