What is the molarity of a solution made by mixing 2.80g of KOH with enough water to make 750. ml of solution?

Molar mass of KOH = (39.0 + 16.0 + 1) g/mol = 56.0 g/mol
No. of moles of KOH = (2.80 g) / (56.0 g/mol) = 0.0500 mol

Volume of the solution = (750 mL) / (1000 mL/L) = 0.750 L
Molarity of the solution = (0.0500 mol) / (0.750 mL) = 0.0673 M

mol(KOH) = 2.80/ (39+16+1)
mol(KOH) = 28. /56 = 0.05

Hence
0.05 = [conc] X 750 mL / 1000 mL
[conc] = 0.05 x 1000 / 750
[conc] = 0.0666 mol dm^-3