Question

a. A student performed Part 3 of the lab. A 360 ppm Ca2+ standard required 22.34 mL of EDTA to reach the end point. Some well water from my grandma’s farm required 15.47 mL of EDTA. By proportionality to the 360 ppm standard, determine the total concentration of Ca2+ (in ppm) in my grandma’s well water?

b. What is the molarity of Ca2+ in the well water? (ppm stands for parts per million. 100 ppm Ca2+ represents 100 grams of Ca2+ in 1,000,000 grams of solution. Assume that the density of the solution is 1.00g/mL).

c. If the daily recommended amount of calcium in an adult’s (age 19 – 50) diet is 1,000 mg, how many liters of water would you need to consume to fully meet your daily calcium needs? (See http://www.nlm.nih.gov/medlineplus/ency/article/002412.htm for reference information.)

Answer 1

EDTA is ethylene diamine tetraacetic acid having molecular formula H₄C₁₀H₁₂N₂O₄. This compound is not very water soluble so the disodium salt is used is used for the estimation of calcium. The formula of disodium salt is :   Na₂H₂C₁₀H₁₂N₂O₄

. Let us suppose that Y stand for C₁₀H₁₂N2O₄. Thus the disodium salt now is Na₂H₂Y.

The part that reacts with calcium ions is H₂Y ^₂- according to the following equation

. H₂Y ^2- + Ca²⁺ → CaY ^2- + 2H⁺

According to this equation moles of EDTA = moles of calcium ions

The concentration of Ca⁺² ions = 360 ppm. This means, 360 mg are present 1L of the solution

Or 0.360g in 1L of the solution

0.360 g of Ca ions = 0.360/40 mol ( 40 is the atomic mass of Ca)

= 9.0x10^-3 mol

Thus the number of moles of EDTA = 9.0x10^-3

Now,9.0x10^-3 moles of EDTA are present in 22.34 ml

As given in the problem, well water from my grandma’s farm required 15.47 mL of EDTA.

22.34 ml of EDTA = 9.0x10^-3 mol of Ca⁺²

Hence 15.47 ml corresponds to 15.47 x 9.0x10^-3/22.34 = 6.23x10^-3 mol of Ca⁺² =  6.23x10^-3x40 = 249.2x10^-3g =249.2 mg = 249.2 ppm

hence,well water from my grandma’s farm has 249.2 ppm of Ca⁺²

b. Molarity of Ca⁺² ions

As stated before 1mg dissolved in 1L of the solution gives 1ppm

Therefore, 249.2 ppm means 249.2 mg are dissolved in 1L of the solution = 0.2492 g

Hence moles of Ca⁺² = 0.2492/40 = 6.23x10^-3

Molarity is the number of moles of solute dissolved per L of the solution

Hence molarity of the solution = 6.23x10^-3 M

c. Well water from my grandma’s farm has 249.2 ppm of Ca⁺²

This means 249.2 mg of  Ca⁺² are in 1 L water

Therefore, 1000 mg will be present in 1000/249.2 = 4.012 L

Hence 4.012 L of water would one need(19-50 age group) to consume to fully meet the daily calcium needs