4.0 g of sodium hydroxide in 250 cm3 of solution contains?

Explanation

Unit of concentration = (moles)/(dm³)
If 250 cm³ solution NaOH contains 4.0 g NaOH
i.e -1cm³ solution NaOH (4.0)/(250) g
∴ 1000 cm³ (1 dm³) will contain (40 x 1000) / (250 x 1) = 16 g/dm³
16 g/dm³ = 16g/dm³ + molar mass NaOH
because no of moles = (mass of substance given) / (molar mass)
∴ (16 g) / (40 g) dm³ = (16 g)/(dm³ x (1 mole)/ (40g) = 0.4 (moles)/(dm³)