Sapling question in chemistry

Question

Ethanol is a common laboratory solvent and has a density of  g/mL. What is the mass, in grams, of  mL of ethanol?

MASS: __________ g

Answer 1

129 mL could be written as:

And 0.789 g/mL could be written as:

So, look what happens to the units when you multiply these:

The mL on top left cancels out the mL in the lower right leaving you with g (grams) and you are looking for grams. So now just multiply and divide the numbers. 129 x 0.789 / 1 / 1 (which, of course is just 129 x 0,789 since dividing by 1 doesn't change anything).

So the mass in grams of the ethanol is 101.781 g. Now, we need to use the proper number of significant figures. There are 3 sig figs in both 129 and in 0.789 so our answer needs to have 3 sig figs making it 102 g

Answer 2

Knowing the definition of density, D = M/V is the key. Cross multiplying gives M = D X V.

0.789 g/mL X 129 mL = 101.781 g, rounds to 102 g.