Question

what would the ph be if the concentration of $\ce{h3o+} = 2 \times 10^{-4}$ mol/l?
\bigcirc 10.30
\bigcirc 3.70
\bigcirc 4.30
\bigcirc -3.70

question 14 (1 point)

a titration involving a very weak acid and strong base was performed. what is the ph at the equivalence point?

\bigcirc ph = 7
\bigcirc ph > 7
\bigcirc ph < 7
\bigcirc the ph cannot be determined.

question 15 (1 point)

polyprotic acids have multiple $k_a$ values. what statement best describes this?

\bigcirc the first $k_a$ is most significant.
\bigcirc all the $k_a$ values are significant for accuracy.
\bigcirc only the first two are significant.
\bigcirc the first is generally very small and they get progressively bigger, as protons become harder to remove.

Explanation:

First Question (pH calculation)

Step1: Recall pH formula

The formula for pH is $pH = -\log_{10} [\ce{H3O+}]$. Here, $[\ce{H3O+}] = 2\times10^{-4}\ \text{mol/L}$.

Step2: Calculate the logarithm

First, find $\log_{10}(2\times10^{-4})$. Using logarithm properties, $\log_{10}(2\times10^{-4})=\log_{10}(2)+\log_{10}(10^{-4})\approx0.3010 - 4=-3.699$. Then, $pH = -(-3.699)\approx3.70$.

When a very weak acid reacts with a strong base, the conjugate base formed is relatively strong. This conjugate base hydrolyzes in water, producing $\ce{OH-}$ ions, which makes the solution basic. So the pH at equivalence point is greater than 7.

For polyprotic acids, the first $K_a$ (dissociation of the first proton) is the largest (most significant) because it is easier to remove the first proton from the acid molecule. Subsequent protons are harder to remove, so their $K_a$ values are smaller and less significant for the overall acidity in most cases.

Answer:

3.70 (the option with "3.70")

Question 14 (Titration pH at equivalence)