chemistry- word problem

Question

There are four naturally occurring isotopes of chromium. Their masses and percent natural abundances are 49.9461 u, 4.35%; 51.9405 u, 83.79%; 52.9407 u, 9.50%; and 53.9389 u, 2.36%.  Calculate the weighted-average atomic mass of chromium.

Express your answer using four significant figures.

Answer 1

Average mass=sum of individual mass*percentage

 

Weighted average means that not all of the mass will exist in equal amounts.  Some isotopic masses exist in more amounts than others.  It's like voting.  The greater number of voters represent the greater percentage.  Therefore, their votes will influence the outcome of an election.

 

For this problem:

 

average mass=49.9461*0.0435 + 51.9405*0.8379 + 52*0.095 + 53.9389*0.0236 or 51.995 u rounded off to 52.0 u.

 

The final answer is closer to 51.9405 because more of this isotope existed than the others (greatest number of votes).