Question

Which of the following solutions has the highest fluorine concentration?

A) 0.05 M CaF2 B) 2.1 mg NaF/L C) a solution having 1.5 mg F- /L D) a solution having 0.06 M F- E) 0.05 M NaF

I dont know how to calculate the concentration of a single element in a compound? Step by step Help would be much appreciated

Answer 1

Lets calculate moles of F fof all given options.

A)

0.05 M CaF2 = 0.05 mol CaF2

1 mol CaF2 contains 2 mol F- therefore,

Moles of F- = 0.05 mol CaF2 x 2 mol F- / 1 mol CaF2= 0.10 mol F-

B)

Mol of NaF = 2.1 mg x (1 g / 1000 mg) x 1 mol NaF/ 41.988 g

= 5.0E-5 mol

Moles of F = mole of NaF = 5.0 E-5 mol

C)

1.5 mg F

Mol of F- = 1.5 mg x 1 g / 1000 mg x 1 mol F/ 18.998 g

=7.89 x 10^-5 mol F-

D)

0.06 M F- = 0.06 mol F-

E)

Moles of NaF=0.05 mol = mol of F-

Among all of these , only Solution (A) , has more F- ions so it has larger concentration.

Answer is “A”