Which has greater Mass 10 mL of steel (Density 7.8 g/mL) or 5 mL of mercury (Density = 13.6 g/mL)?

The steel would have a mass of 78g and the mercury would have a mass of 68g. So the Steel has greater mass.

Explained: This picture I also attached explains that whenever you have an equation dealing with density and volume alone, you will multiply the 2 together to get your answer for mass. And when your dealing with mass and rather volume or density you will divide the mass by whatever the volume or density is. For example if you need to find the density of an object that is 25g and 14ml you will write out this equation 25g/14ml= 1.78 g/ml. And if you need to find the mass of an object that is 27ml and 14g/ml you will write an equation like this 27mlX14g/ml= 378g. I hope this helps with your understanding.

3. Water Vapour!! :)