Question

An element consists of two isotopes. One with a mass of 79.95 amu and an abundance of 29.9%. The second isotope has a mass of 81.95 amu with an abundance of 70.1%. What is the molar mass of this element?

Answer 1

 the molar  mass of the element  is  81.36 g/mol

calculation

step 1 : multiply  each %abundance  of the isotope  by   its mass  number

that is 79.95 x 29.9 =2391

           81.95 x 70.1  = 5745

Step 2: add them together

2390.5 + 5744.7 =8136

Step 3: divide by 100

= 8136/100 = 81.36  g/mol