# Help with some chemistry Q's.

Discussion in 'The Okie Corral' started by USMC Medic, Mar 13, 2010.

1. ### USMC MedicCorpsman Up!

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1) Why is the standard of enthalpy of formation of H2(s) not zero?

2)What volume of a 6.00 M KOH solution must be diluted to 200mL to make a 1.50 M solution of KOH?

3)The value of Delta H for the reaction is -126kJ. The amount of heat that is released by the reaction of 25g of Na2O2 with water is ____kJ.
2Na2O2(s) + 2H2O(l) ---> 4NaOH(s) + O2(g)

It is a shot in the dark, but there are some smart people on here. I need to know how to do these calculations for the quiz on Monday. Thanks for the help.

2. ### AltiDude

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6M/1.5M =4 so 200ml/4 = 50ml of 6M KOH

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3) 20.16 kJ.

4. ### USMC MedicCorpsman Up!

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What equation did you use to find this out?

5. ### devildog2067

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You find the molar mass of Na2O2, convert the amount given in grams to mols. That number divided by 2 (the coefficient in the balanced equation) times the delta H.

Heat is a "product" of the reaction just like anything else, you have to balance it.

EDITED TO ADD: Let me say that I think the reaction "What equation did you use" is exactly the WRONG one. Not a dig against you or anything, but you should be trying to understand the science, not memorize equations. If you really get what it means to balance a given chemical reaction, you wouldn't have to ask that question. Plus of course, the answer is different for any given chemical reaction.

Last edited: Mar 13, 2010
6. ### devildog2067

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Because H2 is not "standard-ly" (s)...

7. ### ElevatedThreatNRA Member

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The Delta H for the reaction is "per mol" of reactants consumed, i.e., for 1 molecular weight of the reactants.

So if you figure out how many molar weights of the reactants there are in the 25 gram weight being asked about, then you can just multiply the number of mols times the Delta H per mole.

Molar weight of Na2O2 = (2 x 23grams/mole) + (2 x 16grams/mole) = 78 grams per mole,

78grams/mole x 2 (because it is 2Na2O2 being consumed in the reaction as written) = 156 grams total

25grams/156grams= 0.1602

0.1602 x (-126kJ) = (-20.19kJ)

-ET