Answer:
0.003981 . . . . moles per liter
Step-by-step explanation:
The concentration of H+ ions in the acid will be ...
10^(-2.4) ≈ 0.003981 . . . . moles per liter
The concentration of H+ ions in the base will be ...
10^-11.2 ≈ 0.000 000 000 006310 . . . . moles per liter
To a few decimal places, the difference is ...
0.003981 . . . . moles per liter
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The two numbers differ by about 9 orders of magnitude, so the value of the difference between the larger and the smaller is essentially the value of the larger number. The smaller one, by comparison, can be considered to be zero (for subtraction purposes).
