The mass of the ocean is about 1.8 1021 kg. if the ocean contains 1.076% by mass sodium ions, na+, what is the mass in kilograms of na+ in the ocean?

In chemistry, if you want to express the amount of a substance out of the total amount, you express it in concentration. There are numerous units of measurement: molarity, molality, normality, mass percentages, volume percentage, or a mix of both. For this problem, the unit used for concentration is in mass percentages. The formula would be

Percentage Concentration = [(Actual Amount of Substance)/(Total amount of all substances)] * 100

Since we are given with the total mass of all the substances in the ocean and the percentage concentration, the only missing information is the actual amount of Na+ in the ocean. Substituting the values:

1.076 = (Amount of Na+ /1.8×10²¹ kg)*100
Amount of Na+ = 1.9368×10¹⁹ kg 

You use the atomic number to find out how many protons and neutrons there are in the atom. you use the atomic mass to find out how many protons and electrons there are in the atom.