A chemist needs 2.00L of an aqueous 0.200M K2Cr2O7 (potassium dichromate) solution. How many grams solid K2Cr2O7 must be weighed out to make the solution

The correct answer is 117.68 g

Explanation:

In order to calculate the mass the chemist has to weigh, we have to multiply the concentration of the solution (0.200 M) by the volume (2.00 L). As the concentration is given in M (mol/L) we first have to convert the concentration from molarity to g/L by using the molecular weight of K₂Cr₂O₇.

From the Periodic Table we know the molar mass (MM) of the constituting elements K, Cr and O:

molecular weight K₂Cr₂O₇ = (2 x MM K) + (2 x MM Cr) + (7 x MM O)

molecular weight K₂Cr₂O₇ = (2 x 39.1 g/mol) + (2 x 52 g/mol) + (7 + 16 g/mol)

molecular weight K₂Cr₂O₇ = 294.2 g/mol

Finally, we can calculate the mass as follows:

mass = 0.200 mol/L x 294.2 g/mol x 2.00 L = 117.68 g