Answer
Approximately $6.708 \times 10^{-5} M$.
Work Step by Step
1. Write the Kps equation:
$K_{ps} = [Ca^{+2}]\times[CO_3^{-2}]$
$[Ca^{+2}] = [CO_3^{-2}] = x$
$K_{ps} = x^2$
2. Find the value of x:
$4.5 \times 10^{-9} = x^2$
$x = \sqrt {4.5 \times 10^{-9}}$
$x \approx 6.708 \times 10^{-5}M $
