Marita E. answered • 01/28/20

PhD in Chemistry/Biochemistry with 7+ years teaching experience

The unit of concentration, ppm, stands for parts per million. Milligrams (mg) per liter (L) are commonly used. To understand why this equals parts per million, we need to remember that there are 1000 milliliters per liter. Assuming that the density of water is 1 g/ml, so:

(1000 ml)(1 g/ml) = 1000 grams of water in 1 liter. Converting that to milligrams (mg):

(1000 grams)(1000 mg/gram) = 1,000,000 mg . This is the mass of 1 liter of water. Hence, the mg of a compound present in 1 liter of solution is its concentration in parts per million.

To solve your problem, you simply need to convert the given concentration, 133.4 g MgBr_{2} in 1.84 L to mg/L:

(133.4 ~~g~~)(1000 mg/~~g~~)= 133,400 mg

Divide that by the number of liters to get your concentration in ppm:

133,400 mg/1.84L = 72,500 mg/L or ppm MgBr_{2}

Since the ratio of bromide to magnesium bromide is 2:1, the concentration of bromide will simply be twice that of MgBr_{2}. I will leave you to perform that simple calculation on your own. Your answer should have 3 sig figs.