please show your work

1.) A beaker is placed on a balance and the balance reads 31.52 g. 10.00 mL of a liquid is added to the beaker and the mass increases to 40.52 g. What is the density of the liquid in g / m * L

2.) What is the mass in grams of 3.81 * 10 ^ 24 molecules of N2O4

3.) How many oxygen atoms are in 7.28g of N2O3

4.) How many moles of oxygen atoms are in 4.08 moles of magnesium phosphate : Mg3(PO4 )2 MW: 262.87 g/mol) ?