How many hydrogen atoms are in 78.4 grams of hydrochloric acid

You should get about 78.4 g H. Convert grams of HCl to moles of HCl by finding total atomic mass of HCl. From there, convert mole of HCl to moles of H which is 1. Last, convert moles of H to g H which is 1g and you should get your answer. Don't forget significant figures. Since you are given 3 in your given information, your answer should have 3 significant figures.