Question

If you want to make 0.5 L of a 0.01 M solution of bromine (Br2) in water, how much bromine would you need? A. 0.8 grams. . B. 0.8 moles. . C. 1.6 grams. . D. 1.6 moles.

Answer 1

The answer is A. 0.8 grams.

The atomic mass of bromine is: A(Br) = 80
The molecule Br₂ consists of 2 Br atoms. Thus, the molecular mass of bromine is: Mr(Br₂) = 2*A(Br) = 2*80 = 160
This means there are 160 grams/l of Br₂ in 1 M.

Let's write a proportion. If 160 grams/l of Br₂ are present in 1 M, how much of Br₂ will be in 0.01 M:
160 g/l : 1 M = x : 0.01 M
After crossing the products:
x = 80 * 0.01 = 1.6 g/l

Let's write another proportion. If there are 1.6 g of Br₂ in 1 liter, how many of Br₂ will be present in 0.5 liter:
1.6 g : 1 l = x : 0.5
After crossing the products:
x = 1.6 * 0.5 = 0.8 g