What is the concentration in moldm-3 of a solution which contains 2.50g of Calcium hydroxide per 500cm3? (Ca=40, O=16, H=1)

Calcium hydroxide is $\text{Ca(OH)}_2$.

Given atomic masses: $\text{Ca}=40$, $\text{O}=16$, $\text{H}=1$.
Molar mass of $\text{Ca(OH)}_2 = \text{Ca} + 2 \times (\text{O} + \text{H})$

Mass of $\text{Ca(OH)}_2 = 2.50 \text{ g}$
Number of moles ($n$) = Mass / Molar mass

Given volume = $500 \text{ cm}^3$
Since $1 \text{ dm}^3 = 1000 \text{ cm}^3$:
Volume ($V$) in $\text{dm}^3 = \frac{500 \text{ cm}^3}{1000 \text{ cm}^3 \text{ dm}^{-3}}$

Concentration ($C$) = Number of moles / Volume

Rounding to three significant figures:

The concentration of the solution is $0.0676 \text{ mol dm}^{-3}$.