How much K2Cr2O7 must be weighed and dissolved in distilled water to prepare 250 mL of a 2 mol/L solution?

1. Determine the molar mass of K_2Cr_2O_7:

M(K_2Cr_2O_7) = 2(39.1) + 2(52.0) + 7(16.0) = 294.2 \text{ g/mol}

2. Calculate the number of moles needed to prepare 250 mL (0.25 L) of a 2 mol/L solution:

n = C \cdot V = 2 \text{ mol/L} \times 0.25 \text{ L} = 0.5 \text{ mol}

3. Calculate the mass of K_2Cr_2O_7 required:

m = n \cdot M = 0.5 \text{ mol} \times 294.2 \text{ g/mol} = 147.1 \text{ g}