The phrase "1 M HCl" describes a concentration, not a fixed number of moles — so the answer depends on how much volume of that solution you have. "1 M" means 1 mole of HCl per liter of solution.
To find moles, use the formula:
Moles = Molarity (M) × Volume (in liters)
Some common examples using a 1 M HCl solution:
- 1 liter (1000 mL) of 1 M HCl → 1 mol × 1 L = 1 mole of HCl
- 500 mL (0.5 L) of 1 M HCl → 1 mol/L × 0.5 L = 0.5 moles of HCl
- 250 mL (0.25 L) of 1 M HCl → 1 mol/L × 0.25 L = 0.25 moles of HCl
- 100 mL (0.1 L) of 1 M HCl → 1 mol/L × 0.1 L = 0.1 moles of HCl
- 10 mL (0.01 L) of 1 M HCl → 1 mol/L × 0.01 L = 0.01 moles of HCl
So if someone says "1 M HCl" without specifying a volume, there's no single numeric answer for moles — you need to know the volume of solution you're working with to calculate it.
A quick tip for calculations: always convert your volume to liters first (divide mL by 1000) before multiplying by the molarity, since molarity is defined in moles per liter.
If you're working backward — say you know you have a certain number of moles and want to find the volume of 1 M HCl needed — just rearrange the formula:
Volume (L) = Moles ÷ Molarity
For example, if you need 2 moles of HCl from a 1 M stock solution, you'd need 2 liters of that solution (2 mol ÷ 1 mol/L = 2 L).
This same logic applies to any molarity value, not just 1 M — always multiply the molarity by the volume in liters to get moles.