Question

A 100.00-mL water sample was adjusted to pH 10 and titrated to the calmagite endpoint with 10.87 mL of 0.0125 M EDTA solution. Then the pH of another 100.00-mL water sample was adjusted to pH 12 and titrated with EDTA to the calcon endpoint. In this second titration, 2.63 mL of titrant was required. Calculate the concentration of Mg2 and Ca2 in the water. Express your answers in mg/L of CaCO3 and MgCO3, respectively.

Answer 1

The mass of Ca²⁺ present in the water sample = 13.23 mg/L.

The mass for the Mg²⁺ in the water sample = 25.06 mg/L.

Step-by-step explanation:

We are given the following set of values which are going to aid in solving this particular question/problem. Thus, the parameters or data or information is given below:

First part:

Total volume of the water sample = 100.00-mL water sample, volume of the EDTA solution = 10.87 mL, the concentration of the EDTA solution = 0.0125 M, the adjusted pH of the water sample with = 10.

Second part:

Total volume of the water sample = 100.00-mL water sample, volume of the EDTA solution = 2.63 mL and the adjusted pH of the water sample with = 12.

Thus, the first thing to do now is to determine the number of moles of the EDTA solution in Ca²⁺ titration which is; 0.0125 x 0.00263 = 3.30 x 10⁻⁵ moles.

The mass of Ca²⁺ present in the water sample = [3.30 x 10⁻⁵ moles × 40.08 ] ÷ 100 mL. = 13.23 mg/L

In 100mL, the number of moles of Mg²⁺ = (total number of moles of EDTA solution used ) - ( number of moles of the EDTA solution in Ca²⁺ titration).

Also, The number of moles of Mg²⁺ = ( 0.0125 x 0.01087) - (3.30 x 10⁻⁵ moles.) = 1.031 x 10⁻⁴ moles.

The mass for the Mg²⁺ in the water sample = 1.031 x 10⁻⁴ moles x 24.305/0.1 = 25.06 mg/L.