Question

mercury has density of 13.53 g/mL. what is the mass of 345mL of mercury?
D=13.53g/mL
Mass=345mL
13.53/345=0.039217391

i just do not think i am doing this right

Answer 1

I agree, you are not doing it right. Look at the units:
mass, you have chosen volume ml.

Density=13g/ml you can do it more precise
volume=345 ml

basic definition
Density=mass/volume
so mass= density*volume
mass= 13g/ml*345ml= xxxx grams