posted by maricela .
Suppose the concentration of sodium in Phoenix tap water is 70 ppm (mass based), then what is the concentration of sodium in milligrams per liter? The density of water is 1 g/mL. The answer units are mg/L.
One easy and valuable conversion unit to memorize is this.
1 ppm = 1 mg/L.
That's all I can help you with since you did not give the substance that was 70 ppm.