You added substance A to a beaker of pure water and obtained a pH reading of 4. Substance A could be a(n) ____________ such as ___________. This solution has ______ than pure water

Question

  Acid; HCl; 1000 times more H+ ions

 

  Acid; HCO3-2; 1000 times more OH-- ions

 

  Acid; NH3; 1000 times more H+ ions

 

  Base; HCl; 30 times more OH-- ions

 

  Base; NH3; 1000 times more H+ ions

 

Answer 1

Hello, Frank,

We'll assume the pH of pure water is 7, which is exactly neutral: There are equal concentrations of both H⁺ and OH^- ions. The full pH scale goes to 14 and is based on water's dissociation constant:

[H⁺] [OH^-] = 1 x 10^-14

A pH of 7 means that both ions are at the same concentration of 1 x 10^-7 M. pH is defined as the negative log of the H⁺ ion concentration. Therefore - log₁₀(1x10^-7) = 7

Since the pH went down to 4 after adding the substance, we can see that the H⁺ concentration went from 1 x 10 ^-7 to 1 x 10^-4. Thus, the [H⁺] increased by 1000 (10³).

Looking at the answer options, we can eliminate any that don't include "1000 times more H⁺ ions. This leaves three options.

1. Acid; HCl; 1000 times more H+ ions
2.  Possible answer, HCl is an acid
3. Acid; HCO3-2; 1000 times more OH-- ions
4.  I'm not sure what the "-2" means, but The [OH] did not increase
5. Acid; NH3; 1000 times more H+ ions
6.  NH3 is a base, it would not lower the pH
7. Base; HCl; 30 times more OH-- ions
8.  Wrong on all levels
9. Base; NH3; 1000 times more H+ ions
10. NH3 is a base

Bob