A brass washer has an outside diameter of 4.50 cm with a hole of diameter 1.25 cm and is 1.50 mm thick. The density of brass is 8599 kg/m3. If you put this washer on a laboratory balance, what will it "weigh" in grams?

We will find the mass from

mass = density x volume

We are told the density and must find the volume from the dimensions given

the volume of the washer will be the area x thickness (remembering to convert all measurements to meters)

if the washer had no hole, its area would be pi (0.0225m) ^2 (remember to convert to meters and to use radius)

the area of the hole is pi (0.00625m) ^2

so the area of the washer is pi[ (0.0225m) ^2 - (0.00625m) ^2] = 1.5x10^-3 m

the volume of the washer is 1.5x10^-3 m x 1.5x10^-3 m = 2.25x10^-6 m^3 (the thickness of the washer is 1.5 mm = 1.5x10^-3m)

thus, the mass of the washer = 8598kg/m^3 x 2.25x10^-6m^3 = 0.0189kg = 18.9 grams