# ap chemistry

Find the grams of NaF added to 300ml of .2M HF to create a buffer with pH=3.5.

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1. Use the Henderson-Hasselbalch equation.
pH = pKa + log [(base)/(acid)]

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2. what is the Ka of HF?

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3. I have a quant book that lists it as Ka = 7.2 x 10^-4

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4. I came up with -5.4 x 10^-4.

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5. What's with the negative sign? Is that a value for Ka you found different from mine. The negative value makes no sense. Or is that the grams NaF to add? Again, the negative sign makes no sense.

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6. I don't know what happened to my response to your post but I made it last night. It showed up on the boards but today it doesn't show UNLESS you click on the main link (the first link) to the question, then all the answers show. At any rate, click on that to get the response. Bascially, I don't understand the negative sign.

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