1.643 g mass of acid has been dissolved
Further explanation
Molarity shows the number of moles of solute in every 1 liter of solute or mmol in each ml of solution

Where
M = Molarity
n = number of moles of solute
V = Volume of solution
Molarity of HCl solution = 0.3 M
Volume of HCl solution = 150 ml=0.15 L
So moles of HCl :

mass of HCl (MW=36.5 g/mol) :
