A0.24 m solution of a weak base a‑ is made. to a limited extent, the a‑ reacts with h2o to form some oh- and some of the corresponding weak acid ha. if the weak acid has a pka of 9.5, what will be the ph of the solution (to the nearest hundredths)? for a hint see the acids and bases handout on canvas.

Solution: The Molarity of solution of weak base is 0.24 M and pK_{a} of weak acid formed by its reaction with water is 9.5.

Let's initially all the weak base reacts with water to form hydroxide ion and weak acid as follows:

A^{-}+H_{2}O\rightarrow OH^{-}+HA

thus, concentration of A^{-}, OH^{-} and HA will become 0, 0.24 and 0.24 respectively.

Reverse reaction will take place as follows:

OH^{-}+HA\rightarrow A^{-}+H_{2}O

The change in concentration takes place, let's the change be x thus, concentration of OH^{-}, HA and A^{-} will become 0.24-x, 0.24-x and x respectively.

For the reaction, expression for acid dissociation constant will be:

K_{a}=\frac{[A^{-}]}{[HA][OH^{-}]}

Putting the values of concentration,

K_{a}=\frac{[x]}{[0.24-x][0.24-x]} (1)

From the pK_{a} of weak acid, acid dissociation constant can be calculated as follows:

pK_{a}=-logK_{a}

thus,

K_{a}=10^{-pK_{a}}

=10^{-9.5}

=3.16\times 10^{-10}

Putting the value in equation (1),

3.16\times 10^{-10}=\frac{[x]}{[0.24-x][0.24-x]}

Here, the value of K_{a} is very small so x can be neglected from denominator, thus,

3.16\times 10^{-10}=\frac{[x]}{[0.24][0.24]}

On solving,

x=4.26\times 10^{-6}

Concentration of hydroxide ion will be:

[OH^{-}]=0.24-x=0.24-4.26\times 10^{-6}=0.2399

now, pOH will be:

pOH=-log[OH^{-}]

=-log(0.2399)

=0.62

Use the following equation to calculate pH,

pH+pOH=14

thus,

pH=14-pOH

=14-0.62

=13.38

Thus, pH of solution is 13.38

in scientific notation, the value of the  speed of light in a vacuum

  will be written as 2.995 * 10^8. when this value is represented in the form of

  a * 10^b, then, a = 2.995 and b = 8.

the value of the speed of light in a vacuum given above can also be rounded up to approximately 300000000. in this case, the scientific notation will be written as 3 *10^8.

answers: to make sure that all of the broken pieces of tablet were added to the water, to allow for fair comparison of all three trials, with different particle sizes to avoid experimental error