#### Answer

a.) 17.007
b.) 19.023
c.) 140.97
d.) 172.79

#### Work Step by Step

a.) To find the molecular mass, add the atomic masses of oxygen (15.994) and hydrogen (1.00794) to get 17.00734. Then, express the answer in five significant digits and you will get 17.007.
b.) To find the molecular mass, you must first multiply the atomic mass of hydrogen (1.00794) by 3, because there are three hydrogens. Then, add this value to the atomic mass of oxygen (15.9994) to get 19.02322. When this value is expressed in five significant digits, you will get 19.032.
c.) To find the molecular mass, you must first multiply the atomic mass of fluorine (18.9984) by 6, because there are six fluorines. Then, add this value to the atomic mass of aluminum (26.98154) to get 140.97194. When this value is expressed in five significant digits, you will get 140.97.
d.) To find the molecular mass, you must first multiply the atomic mass of chlorine (35.453) by 4, because there are four chlorines. Then, add this value to the atomic mass of phosphorous (30.97376) to get 172.78576. When this value is expressed in five significant digits, you will get 172.79.