Give the ions formed when the following compounds dissociate: I. KOH II. Pb(NO3)2 III. Na2CO3

Given atomic masses: $\text{Na} = 23$, $\text{C} = 12$, $\text{O} = 16$.

$$\text{Molar mass of } \text{Na}_2\text{CO}_3 = (2 \times 23) + 12 + (3 \times 16)$$ $$= 46 + 12 + 48 = 106 \text{ g/mol}$$

The formula for moles is: $\text{Moles} = \text{Concentration} \times \text{Volume}$.

$$\text{Moles of } \text{Na}_2\text{CO}_3 = 0.2 \text{ mol/dm}^3 \times 0.250 \text{ dm}^3$$ $$= 0.05 \text{ mol}$$

The formula for mass is: $\text{Mass} = \text{Moles} \times \text{Molar mass}$.

$$\text{Mass of } \text{Na}_2\text{CO}_3 = 0.05 \text{ mol} \times 106 \text{ g/mol}$$ $$= 5.3 \text{ g}$$

The mass of sodium trioxocarbonate (IV) needed is $\boxed{\text{5.3 g}}$.