Review how to calculate the mass of a liquid from its volume and density. Density is mass per unit volume:

density = mass / volume

You can rewrite the equation to solve for mass:

mass = volume x density

Density of liquids usually is expressed in units of g/ml. If you know the density of a liquid and the volume of the liquid, you can calculate its mass. Similarly, if you know the mass and volume of a liquid, you can calculate its density.

Calculate the mass of 30.0 ml of methanol, given the density of methanol is 0.790 g/ml

mass = volume x density

mass = 30 ml x 0.790 g/ml

mass = 23.7 g

In real life, you can usually look up the density of commons liquids in reference books or online. While the calculation is simple, it's important to cite answer using the correct number of significant digits.