If 75 mL of a hard water sample required 23.00 mL of 0.200 M EDTA, calculate the total hardness.

Question:

If 75 mL of a hard water sample required 23.00 mL of 0.200 M EDTA, calculate the total hardness.

Water Hardness

Hardness in water is a quantification of the mineral content in water. In practice it is measured as the concentration of divalent ions which can be measured by titration with EDTA. Typical units of hardness are mmol/L and mg/L which also corresponds to units of ppm.

Answer and Explanation:

The reaction between either calcium or magnesium ions and EDTA is a 1:1 chelation reaction.

{eq}M^{2+} + EDTA^{4-}\rightarrow [M--EDTA]^{2-} {/eq}

Every mole of EDTA chelates with 1 mol of metal, so to find the moles of metal in the original volume, we simply need to calculate the moles of EDTA consumed.

{eq}0.023\;L\times \dfrac{0.2\;mol}{L}=0.0046\;mol {/eq}

{eq}\dfrac{0.0046\;mol}{0.075L}=0.0613\;mol/L {/eq}

Water hardness is more commonly reported in mmol/L, so the total hardness is 61.3 mmol/L.


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