Calcium ion present in 250-mL of tap water is precipitated with the addition of

Question

Calcium ion present in 250-mL of tap water is precipitated with the addition of sodium oxalate to torn calcium oxalate, CaC_2O_4. An excess of precipitating agent is added to 250-mL of water. The resulting precipitate is filtered, washed, and heated in a crucible to form the more stable calcium oxide, CaO. The mass of the empty crucible was 25.6005 g. The mass of the crucible with the calcium oxide precipitate was 25.7627 g. a) Calculate the grams of Ca^2+ in the water sample. b) Calculate the % Ca^2+ in the water sample. c) Calculate the ppm Ca^2+ in the water sample. d)Water "hardness" is calculated in units of mg CaCO_3 per 100-ml water. Calculate the hardness of the water in units of mg CaCO_3 per 100-mL water. How does this water rate on the water hardness scale (look up on the internet).

Explanation / Answer

c)

ppm = mg of specie / liters of solution

ppm = (0.1156*10^3) / (0.25) = 462.4 ppm

d)

mg CaCO3 per 100 mL

relate Ca+2 = CO3-2

so

1 mol of Ca+2 --> 1 mol of CacO3

so..

0.00289 mol of Ca+2 --> 0.00289 mol of CaCO3

this i per V = 250 mL

so

per 100 mL

100/250*0.00289 = 0.001156 g/mL

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