Molarity is the number of moles of solute in 1 L of solution. the mass of NaCl added - 20.45 g number of moles of NaCl - 20.45 g / 58.5 g/mol = 0.350 mol volume of the solution is 700.0 mL since molarity is the number of moles in 1000 mL and if 700.0 mL contains - 0.350 mol therefore 1000 mL contains - 0.350 mol / 700.0 x 1000 = 0.500 mol hence molarity of solution is 0.500 M
Ok so all are in the alkali family also they are all metals and in group 1 but I think the one that might reacts quickest is Li because it has less AMU
