Enable the mhchem extension as described in the KaTeX

[defencedog]

I am using KATEX to maximum owing to my chemical engineering background & congo perhaps is limiting me
From the official hugo documentation I am quoting

As shown in Step 2 above, MathJax supports chemical equations without additional configuration. To add chemistry support to KaTeX, enable the mhchem extension as described in the KaTeX documentation.

And currently this syntax do not work

$$C_p[\ce{H2O(l)}] = \pu{75.3 J // mol K}$$