Assume that acid rain has lowered the pH of a particular lake to ...

[Hawke]

Assume that acid rain has lowered the pH of a particular lake to pH 4.0. What is the hydroxyl ion concentration of this lake?
A) 1 × 10-10 mol of hydroxyl ion per liter of lake water
B) 1 × 10-4 mol of hydroxyl ion per liter of lake water
C) 10.0 M with regard to hydroxyl ion concentration
D) 4.0 M with regard to hydroxyl ion concentration
E) 1 × 10-4 mol of hydroxyl ion per liter of lake water and 4.0 M with regard to hydrogen ion concentration

[Celeste]

The answer I found is A. Let me know if you need more help!