Question

Give a solution of $1$M acid, HA, with $K_a=1\times {10}^{-6}$.
What is the $H_3{O}^{+}$ concentration? (Assume$\left[HA\right]=1,\left[H_3{O}^{+}\right]=x,\left[A^{-}\right]=x$).

• A. $1\times {10}^{-5}$ M
• B. $1\times {10}^{-4}$ M
• C. $1\times {10}^{-2}$ M
• D. $1\times {10}^{-3}$ M
• E. $0.9\times {10}^{-3}$ M

Answer 1

Answer: (D)

$1\times {10}^{-3}$ M

$K_a=\frac{\left[H_3{O}^{+}\right]\left[A^{-}\right]}{\left[HA\right]}1\times {10}^{-6}=\frac{x\times x}{1}x=\sqrt{1\times {10}^{-6}}=1\times {10}^{-3}M=\left[H_3{O}^{+}\right]$