ppm is parts per million. concentration of a solution can be expressed in parts per million, this is equivalent to mg/L.
therefore concentration in ppm is how many mg have been dissolved in 1 L of solution.
There are 2.2 mg of Pb in 0.5 L solution Therefore in 1 L of solution - 2.2 /0.5 x 1 = 4.4 mg/L
Therefore concentration is 4.4 ppm
