What is the molarity of a solution that contains 0.400 mol HCI in 9.79 L solutions

Molarity's formula is known as: Molarity (M) = moles of solute/liters solution.

In this case we are already given moles and liters so you just have to plug the numbers into the equation.

0.400 mol HCL/9.79L solution=0.040858M

If you were to use scientific notation, the answer will be: 4.1*10^-2, but otherwise, you can just use the decimals above and round appropriately as you see fit.