1. Molarity (M)

* Definition: Molarity is the number of moles of solute dissolved in 1 liter of solution.

* Formula: Molarity (M) = moles of solute / liters of solution

* Example: A 1 M solution of sodium chloride (NaCl) contains 1 mole of NaCl dissolved in 1 liter of water.

2. Percent by Mass (%)

* Definition: Percent by mass is the mass of the solute divided by the mass of the solution, multiplied by 100%.

* Formula: Percent by mass (%) = (mass of solute / mass of solution) x 100%

* Example: A 10% by mass solution of sugar in water means that 10 grams of sugar are dissolved in 100 grams of solution (sugar + water).

Let me know if you'd like to explore other ways of expressing concentration!