Hardness in groundwater is due to the presence of metal ions, primarily Mg2 and

Question

Hardness in groundwater is due to the presence of metal ions, primarily Mg2 and Ca2 . Hardness is generally reported as ppm CaCO3. To measure water hardness, a sample of groundwater is titrated with EDTA, a chelating agent, in the presence of the indicator eriochrome black T, symbolized here as In. Eriochrome black T, a weaker chelating agent than EDTA, is red in the presence of Ca2 and turns blue when Ca2 is removed.

A 50.00-mL sample of groundwater is titrated with 0.0600 M EDTA. Assume that Ca2 accounts for all of the hardness in the groundwater. If 11.20 mL of EDTA is required to titrate the 50.00-mL sample, what is the hardness of the groundwater in molarity and in parts per million of CaCO3 by mass?

Explanation / Answer

Answer:
Hi student, very simple problem but more confusing any way I will explain in details.
First step- To find the moles of EDTA
Moles of EDTA= Molarity of EDTA X volume of EDTA in litres
=0.0600M X (11.20/1000)L
=6.72 X 10-4

Moles of EDTA = 0.000672moles
The end point is where the moles of EDTA and moles of Ca2+ have to be the same.
Hence, moles of Ca2+ is also 0.000672 moles.
There's a 1:1 ratio of Ca2+ to CaCO3, so CaCO3, also have 0.000672 moles.
Step –II Find molarity
Molarity= Moles/ volume in lit.
=0.000672 / 0.05 L (50 ml / 1000=0.05L)
=0.01344M CaCO3

Molarity = moles/L PPM = mg/L

so (0.01344 mole CaCO3/1L)(100.1g/1 mole CaCO3)(1000mg/1g)((1ppm x 1L)/1mg)
= 1344 ppm

Note (1L=1000 ml)

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