To identify metal M in an unknown metal carbonate, MCO₃, the following procedure is carried out: Step 1: 0.198 g of IMPURE MCO₃ is reacted with 25 cm³ of 0.4 mol·dm⁻³ nitric acid, HNO₃(aq) - NSC Physical Sciences - Question 7 - 2023 - Paper 2

To calculate the number of moles of Ba(OH)₂ reacted, we start with the equation:

1. The number of moles of HNO₃ used is:

   $n(HNO_3) = 0.4 ext{ mol/dm}^3 imes 0.025 ext{ dm}^3 = 0.01 ext{ mol}$

2. According to the balanced equation, 2 moles of HNO₃ react with 1 mole of Ba(OH)₂. Thus, the moles of Ba(OH)₂ reacted is:

   n(Ba(OH)_2) = rac{n(HNO_3)}{2} = rac{0.01}{2} = 0.005 ext{ mol}

To determine the pH after Step 1:

1. Identify the moles of excess HNO₃:

   • The moles of HNO₃ initially present is 0.01 mol, and the moles that reacted with MCO₃ is:

   • Assuming the carbonate reacts with 0.002 mol,

   $ext{Excess} = n(HNO_3) - n(MCO_3) = 0.01 - 0.002 = 0.008 ext{ mol}$

2. The total volume of the solution after Step 1 is 25 cm³ + 20 cm³ == 45 cm³ = 0.045 dm³.

3. Concentration of H⁺ ions is:

   [H^+] = rac{0.008 ext{ mol}}{0.045 ext{ dm}^3} = 0.178 ext{ mol/dm}^3

4. The pH is calculated as:

   $$$$

ightarrow pH ext{ is about } 0.75$$

From the calculations, we found the mass of MCO₃ is 0.168 g after considering the purity:

1. If 85% purity is given, then the molar mass of the compound can be evaluated:

   ext{Purity} = rac{ ext{mass of pure MCO}_3}{ ext{mass of sample}} imes 100

   Rearranging gives us:

   $ext{mass of pure MCO}_3 = ext{Purity} imes 0.198 ext{ g} = 0.168 ext{ g}$

2. The molar mass of MCO₃ can be determined based on the formula weight. Therefore:

   • If we identify that the molar mass is approximately 84 g/mol, then metal M would correspond to the metal with atomic mass 60 g/mol:

   Therefore, the identified metal M is likely Calcium (Ca).