Question

Three aliquots of a sample solution containing Ca2+ were analyzed using the procedure given above. The solution temperature was 17.0 °C during the pipetting step, and the pipet had been previously calibrated to deliver 9.9804 mL. The resulting CaC2O4·H2O precipitates were found to have an average, uncorrected mass of 0.19118 g. Calculate the calcium concentration in the original sample, corrected to 20 °C. (Note that CaC2O4·H2O contains 27.42961 %Ca. The density of CaC2O4·H2O is about 2.2 g/mL.)

Answer 1

Step 1 , -

Calculate ,-

a) Error in measurement of volume pipetted = 10 - 9.9804 = 0.0176 ml

b) weight of CaC2O4.2H2O which should have been precipitated from 10 ml. of sample solution at 20^o C

......................................................................................................................= 0.19118 + ( 0.0176 x 2.2 )

..................................................................................................................... = 0.19118 + 0.03872

.......................................................................................................................= 0.2299 gms

Step 2 , -

Since percentage calcium in CaC2O4.2H2O is given as 27.42961 %

therefore , weight of Ca in 10.00 ml solution at 20^o C = ( 27.42961 x 0.2299 ) / 100

......................................................................................= 0.0630606 gms.

Step 3, -   

Calculate calcium concentration of original solution at 20^o C as = ( 0.0630606 x 100 ) / 10

...................................................................................... = 6.30606 % by weight

..................................................................................or, = 6.31 %

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