How many molecules of liquid ethanol are in 1.00 mL of ethanol, given its density is 0.789 g/mL?

Mass = volume * density = 1.00 mL * 0.789 g/mL = 0.789 g

The relative molecular mass of ethanol can be calculated by adding the relative atomic masses of all the atoms within its structure (C2H5OH):

(12.011 g/mol * 2) + (15.999 g/mol * 1) + (1.0079 g/mol * 6) = 46.068 g/mol

2 Carbon 1 Oxygen 6 Hydrogen

This can be used to find the number of moles of ethanol:

moles = mass / rmm = 0.789 g / 46.068 g/mol = 0.0171 mol (3 sig. fig.)

Using Avogadro's constant, the number of molecules within 1 mole of substance, the number of molecules within 1 mL of ethanol can be found:

no. molecules = moles * Avogadro constant

= 0.0171 mol * (6.022*10^23 1/mol)

= 1.03*10^22 molecules