Final answer:
To find the concentration of NaOH in mol/dm³, divide the mass of NaOH (3.5 g) by its molar mass (40.0 g/mol) to find the moles, and then divide by the volume (2.5 dm³). The result is 0.035 mol/dm³. To find the concentration in g/dm³, divide the mass by the volume to get 1.4 g/dm³.
Step-by-step explanation:
To calculate the concentration of NaOH in mol/dm³ (molarity) when 3.5 g of sodium hydroxide is dissolved in 2.5 dm³ of water, we use the molar mass of NaOH (40.0 g/mol) and the definition of molarity. First, we convert the mass of NaOH to moles:
3.5 g NaOH × (1 mol NaOH / 40.0 g NaOH) = 0.0875 mol NaOH
Then, we divide the number of moles by the volume of the solution in dm³ to get the molarity:
0.0875 mol NaOH / 2.5 dm³ = 0.035 mol/dm³
To convert this to g/dm³, we simply use the initial mass of NaOH:
3.5 g NaOH / 2.5 dm³ = 1.4 g/dm³
Thus, the concentration of the NaOH solution is 0.035 mol/dm³ or 1.4 g/dm³.