biochem amino acid question.

[RICEman]

How do you calculate the [HA] and [A-] forms of cysteine required to produce an overall 0.25M solution at pH of 7.8?
I figured the pI = 5.145.
I also solved the pOH=14-7.8=6.2



The text says that the COO- = 1.92, NH3 = 10.70, and Sulfhydryl group = 8.37

I've been going in circles for this question......

Also, based on the above, how would the structure look like in its "HA" and "A" forms?

Thanks.

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