A chemist oxidises 4.60 g of ethanol with PCC in dichloromethane and obtains ethanal in a yield of…

Step 1. $n = \dfrac{4.60\ \text{g}}{46.07\ \text{g/mol}} = 0.0998\ \text{mol} = 99.8\ \text{mmol}$.

Step 2. One mole of alcohol gives one mole of aldehyde, so the theoretical mass is $0.0998\ \text{mol} \times 44.05\ \text{g/mol} = 4.40\ \text{g}$.

Step 3. Actual mass $= 4.40\ \text{g} \times 0.82 = 3.61\ \text{g}$.

Why not dichromate? Hot acidified dichromate oxidises ethanal on to ethanoic acid. Unless the ethanal is distilled out as it forms, most of it is lost.