A 2.0x10 -3 M solutions of Na 2 H 2 EDTA2H 2 O (FM 372.24) was used to titrate 5

Question

A 2.0x10-3M solutions of Na2H2EDTA2H2O (FM 372.24) was used to titrate 5.000 mL of a seawater sample dissolved in 50 mL of deionized water. Eriochrome Black T was used as an indicator. The endpoint was observed after the addition of 37.63 mL of titrant. Then, a second sample was prepared. This time excess sodium hydroxide was added to 3.000 mL of the sample, which was then titrated with the 2.0x10-3M EDTA. The endpoint was observed after 8.00 mL of titrant was added. Based on these two titrations, what is the [Ca2+] and [Mg2+] in the seawater sample?

Explanation / Answer

M1V1 = M2V2

[Mg2+] * 37.63 mL = 2 x 10^-3 M * 5 mL

[Mg2+] = 2.66 x 10^-4 mol / L

[Ca2+] = 2 x 10^-3 M * 3 mL / 8 = 7.5 x 10^-4 mol/L

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