# Significant figures conflict [duplicate]

So I am asked to solve this question from Silberberg:

Use Appendix C to calculate $[\mathrm{H_2S}]$, $[\mathrm{HS}^-]$, $[\mathrm{S^{2-}}]$, $[\mathrm{H_30}^+]$, $\mathrm{pH}$, $[\mathrm{OH}^-]$, $\mathrm{pOH}$, in a $0.10 \mathrm{M}$ solution of the diprotic acid hydrosulfuric acid.

In Appendix C (Equilibrium Constants for Selected Substances at $298 \mathrm{K}$),

$\mathrm{H_2S}$ has a $K_a$ of $9 \times 10^{-8}$ (which has one sig.fig.)

I answered it with one sig.fig. because $K_a$ has one sig.fig. but my teacher told me that $K_a$ is a constant (I did not understand why and he told me that the burden of proof is up to me) so it should be two sig.fig.

I checked the Answers appendix of the book, and all of the answers were in one sig.fig except $[\mathrm{H_2S}]$, which had $0.10 \mathrm{M}$.