Question

please can you solve this please

2 Determine the colour change at the endpoint of the titration Calculate the weight of the unknown sample to produce a 250 mL Ca2+ solution at about 0.005 M. Assume that the weight percent of CaCO3 in the unknown is 50 %.

Answer 1

2. Determine the colour change at the endpoint of the titration.

Ca²⁺ + oxalate and titrate with KMnO4 -------> end point = purple colour

3. Calculate the weight of the unknown sample to produce 250 mL Ca²⁺ solution at about 0.005 M. Assume that the weight percent of CaCO₃ in the unknown is 50%.

Unknown solution has 50 % by mass of CaCO₃

100 g solution has 50 g CaCO3

Moles of Ca²⁺ = Moles of CaCO₃

= mass/ molar mass

= 50 g / 100.09 g/mol

= 0.50 moles

So, moles of Ca²⁺ = 0.50 moles in 100 g solution

Assuming density of solution ~ 1 g/ mL, we have 0.50 moles of Ca²⁺ in 100 mL solution
(or) 5.0 moles in 1 L solution.

Hence, [Ca²⁺] = 5.0 M

We require a molarity of 0.005 M

Volume = 250 mL

Using law of dilutions:

C₁V₁ = C₂V₂

5.0 M x V₁ = 0.005 M x 250 mL

V₁ = 0.005 M x 250 mL / 5.0 M

V₁ = 0.25 mL

We need, 0.25 mL of unknown sample or 0.25 g sample.