a) What molecule makes up the majority of 0.020 M LiF (aq)?

Question

(M stands for molarity - moles/L). Hint: no math needed.

b) What element makes up the majority of atoms, by mass?

Answer 1

0.02 M LiF(aq) suggests that you have a solution of LiF in water. Thus, you would have

Li⁺(aq), F^-(aq) and H₂O(l) present. And LiF isn't really considered a molecule, but rather is considered a formula unit.

My answer would be that H₂O is the molecule that makes up the majority of LiF(aq).

By mass (assume 1 L of solution):

0.02 mol Li = 0.02 mol x 6.94 g/mol = 0.139 g

0.02 mol F = 0.02 mol x 19 = 0.38 g

~ 1L H2O = ~1000 g

H2O is ~ 11% H and ~ 89% O

So, Oxygen would be the element that makes up the majority of atoms, by mass.

Answer 2

Hello, Arianna,

The question does not specify what is meant by "majority." Majority of atoms, or majority of mass. The concentration makes no difference is answering either of these questions. It is there to confuse us.

If we assume majority by numbers of atoms, the answer in neither - Li and F come in pairs, so they are equal to each other in terms of count.

In terms of mass, which is what I believe the question really wants to know, fluorine is the one with the majority of mass. The molar mass of Li is 7 while F is 19. These can be written as grams/mole. So regardless of the concentration, since Li and F come in pairs, F will constitute the majority of the mass, regardless of concentration.

Bob