The average atomic mass of sodium is 22.99. The average atomic mass of chlorine is 35.451. NaCl has one of each, which adds up to atomic mass of 58.441 atomic mass units. To find out how much of that is sodium, do 22.99รท58.441. you get 0.3933. So you know in any amount of NaCl, sodium makes up 39.33%. 39.33% of 50 is 19.665 g, so the answer is 19.665 g.