Question

Given [H3O+] = 1.00 x 10-7M
Is this acidic, basic or neutral?
How do you know?​

Answer 1

Answer:

This solution is neutral.

Assumption: the solution is under room temperature, such that $K_w = 10^{-14}$.

Explanation:

For a solution in water,

$\mathrm{[H_3O^{+}] \cdot [OH^{-}]} = \mathnormal{K_w}$.

In other words, if $K_w$ is given, knowing the concentration (in $\rm M$, or equivalently $\rm mol\cdot L^{-1}$) of either $\rm H_3O^{+}$ or $\rm OH^{-}$ will imply the concentration of the other ion.

Under room temperature,

$K_w \approx 10^{-14}$.

The question states that for this solution,

$\mathrm{[H_3O^{+}]} = \rm 1.00\times 10^{-7}\; M$

As a result, the concentration of $\rm OH^{-}$ in this solution will be

$\displaystyle \frac{K_w}{\mathrm{[H_3O^{+}]}} = \frac{10^{-14}}{1.00\times 10^{-7}} = 10^{-7}$.

• A solution is acidic if $\mathrm{[H_3O^{+}] > [OH^{-}]}$.
• A solution is basic if $\mathrm{[H_3O^{+}] < [OH^{-}]}$.
• A solution is neutral if $\mathrm{[H_3O^{+}] = [OH^{-}]}$.

In this case, $\rm [H_3O^{+}] = 10^{-7}\; M = [OH^{-}]$. In other words, this solution is neutral.