stock solution makeup

Question

To prepare 500 mL of stock standard solution of 1000 mg/L chromium using potassium dichromate one would:

Select one:

a. Dissolve 1.4145g potassium dichromate and make up to 0.500 L final volume.

b. Dissolve 0.5000g potassium dichromate and make up to a final volume of 0.500 L.

c. Add 500 mg potassium dichromate to 0.500 L water.

d. Dissolve 0.3535g potassium dichromate and make up to 0.500 L final volume.

e. Add 1.4145 g potassium dichromate to 0.500 L water.

Answer 1

The ratio of K₂Cr₂O₇ to Cr in the compound is given by summing the atomic weights of the elements of K₂Cr₂O₇ taking account of the number of atoms of each element and dividing by 2 x the atomic weight of Cr (51.996). The gives 2.829 g (or mg) K₂Cr₂O₇ for every g (or mg) Cr. As the desired concentration is 1000mg/L and you want to make 500 mL, (0.5L) multiply 2.829 x 500 mg = 1414.5 mg K₂Cr₂O₇ needed. The is equivalent to 1.4145 g of K₂Cr₂O₇ dissolved in enough solvent to make 500 mL of solution; therefore, choice a is the answer.