The atomic number of an element is 8. How many electrons will it gain to form a compound with sodium?

A sample of oxygen contains two isotopes of oxygen with masses 16u and 18u respectively. The proportion of these isotopes in the sample is 3 : 1. What will be the average atomic mass of oxygen in this sample?

Which one of the following is a heterogeneous mixture?

What is the formula mass of anhydrous sodium carbonate? (Given, that the atomic masses of sodium, carbon and oxygen are 23u, 12u and 16u respectively).