CHEM

posted by tom

If 100g of NaOH are reacted with 85.00g H3PO4, how many grams of Na3PO4 can be made?
Make sure the final answer is in the correct number of sig figs.

3NaOH + 1H3PO4 ---> 1Na3PO4 + 3H2O

I got 100g Na3PO4

1. Scott

you do remember that the coefficients in the reaction equation are for moles ... not mass

2. tom

I know that.

3. Scott

looks good

4. tom

5. DrBob222

No, the answer is not 100 g. Look at what Scott asked about moles. And your response about grams appears that you think you can go from grams to grams without going through mol ratio. You can't.

6. tom

dude i converted from grams to moles to moles back to grams

7. tom

and the answer key says it's 100g

8. DrBob222

The answer is not 100 g. If the answer key says it is 100 g then the answer key is not right. If you will post your work I will find the error you are making. And yes, I understand all about converting grams to mols and back to grams. I suspect what you have not done correctly is to convert from mols NaOH to mols Na3PO4 properly but I don't know that. By the way, did you identify the limiting reagent?The correct answer is 137 g to three sig fig.

9. Scott

yes , the 137 can be calculated , but the note about significant figures is, well, significant

how many sig fig in the NaOH mass data?

that's why 137 becomes 100

10. DrBob222

I see that now and thanks for pointing it out to me. I wondered at the time why you had agreed with 100. However, 100 still isn't right, is it? Technically, shouldn't the answer be 1E2 grams since there is only 1 sig fig in 100 g? Therefore,we should quote the answer as 1E2 grams Na3PO4 (to 1 sig fig). Thanks again.

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