If you mix 4.5 g of hcl in enough water to make 0.9 l (900 ml of solution, what is the molarity of the solution?

We first need to convert grams of hydrogen chloride to moles.

molarity = moles of solute/liters of solution

To convert from grams to moles, we need to divide by it's molecular mass.

4.5 g HCl * (1 mole HCl / 36.46 g HCl) = 0.123 moles HCl

0.12 moles/0.9 L = 0.13 M HCl

If they gave you the volume as 900 mL, then that would be 3 sig figs. And the sig figs would be limited to 2 because the mass. If, however, they gave you the volume as 0.9 L, then that would be 1 sig fig. And the final answer would be limited to to 1 sig fig.