to solve the concentration, it is equal to the mass of the calcium ions divided by the total mass of the solution.
C = mass of ions / mass solution
C = ( 8.5 x 10 ^-3 g ) / 490 g
C = 1.7 x 10^-5
since the choices are in ppm, multiply it by 10^6
C = (1.7 x 10^-5 ) 10^6
C = 17 ppm
