A beaker containing 1000. mL of water at 20°C weighs 998 g. What is the density of water at 20°C?

Question

The density of a substance is its mass divided by its volume. A beaker containing 1000. mL of water at 20°C weighs 998 g. What is the density of water at 20°C?

Round your answer appropriately.

density=

 _____g/mL

Answer 1

Hi Deanna R.,

The standard answer and route to the answer would be, since you want to end up with g/mL, to divide the mass in grams by the volume in milliliters and round appropriately. Hence, 0.998 g/mL . The volume has allegedly 4 significant digits, and the mass, 3 significant digits. However, that's a little deceptive isn't it, since the uncertainty of each is ~ +/- 0.5 unit per 1000 units, hence the 3 significant digits of the 998 are each a little more significant than the 4 digits of 1000 are.

Now the fun part! One would never put 1000. ml water in a beaker for weighing! You would keep it in a class A volumetric flask, or more particularly, in a pycnometer, to facilitate its thermal equilibration. If you DID measure it out in a volumetric flask, you would NOT transfer it to a beaker for weighing, since you could never transfer all of it, and it would be evaporating as you transferred it anyway..

And one more thing: the problem states that the beaker with the water weighs 998 g. Logically, that includes the mass of the beaker, too. Perhaps the questioner meant that the weight of the water in a tared beaker is 998 g. Perhaps.

-- Cheers, -- Mr. d.