Use the following scenario for the next four questions: An unknown water sample
ID: 887622 • Letter: U
Question
Use the following scenario for the next four questions:
An unknown water sample is tested to determine its hardness as measured in ppm of calcium carbonate. It is found that in a titration with the disodium salt of EDTA, 45.8 mL of a 0.00105 M EDTA solution is required to bind all of the calcium and magnesium ions in 25.0 mL of the water sample.
If all of the moles (as determined in the previous problem) of calcium and magnesium were moles of calcium carbonate, what mass of calcium carbonate is in the 25.0 mL sample of water?
Assuming the density of water is 1.00 g/mL, calculate the ppm of calcium carbonate in the water sample.
Explanation / Answer
From the data given
1 mole of Ca2+ = 1 mole EDTA
No of moles of EDTA = (45.8/1000)*(0.00105)
= 0.00004809 mole
= 4.809*10^(-5) mole
No of moles of Ca2+ = 4.809*10^(-5) mole
molarMASS of CaCo3 = 100.0869 g/mol
MASS of CaCo3 in 25.0 mL sample of water = 4.809*10^(-5)*100.0869
= 0.004813 grams
ppm = (0.004813/25)*10^6 = 192.52 ppm