Heat of _____: Q=mcT versus Q=mH

Discussion in 'DAT Discussions' started by pandalove89, Aug 6, 2011.

1. pandalove89 7+ Year Member

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Okay so I understand that the "m" in Q=mc(delta)T is mass, but isn't the m in
Q = m(delta)H , moles instead of mass? I swear that I saw Chad use moles in when calculating heat of fusion, melting, etc.

However, when I tried to solve a problem in kaplan they said it was mass instead of moles....

ps: sorry for the overload of questions. test is soon and I'm freaking out

2. needzmoar

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it just depends on the units. if H is given per mole you do H*n . If it's given per mass then you do H*mass

3. JohnnnyD 5+ Year Member

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I actually had this same question not long ago. with the Q=mc(delta)T equation you are going to have m=mass. for the Q=mH, you need to look at the question. when they give you the deltaH in the question, it will either be related to moles or grams, so look at the units. if its in moles, use m=moles, and if its in grams, use m=mass.

4. yappy 10+ Year Member

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Looking at units is the correct thing to do, but for clairity - Most calculations I do while researching is using contants with mols. Also, most practice questions I"ve done use mols for calculating q of phase change.

5. OP

pandalove89 7+ Year Member

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so if its asking for moles, then both Q=mcT and Q=mH is moles? Not just Q=mH?

6. needzmoar

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No panda, just look at the units of the constants that are given. It really depends on the units. Copy and paste the problem and I'll show you what I mean.

edit: also, I don't understand what you're trying to ask in the quote.

edit#2 : Here's a sample problem to illustrate my point.

How much energy is released when 100.0 g of diethylether is cooled from 53.0 °C to 10.0 °C?
Boiling point = 34.5 °C; &#916;Hvap = 351 J/g; specific heat of liquid = 3.74 J/g K; specific heat of gas = 2.35 J/g

Do you see that the units of &#916;Hvap is in J/g ? That means you need to multiply H by mass. If you blindly multiply H by mols you will get the wrong answer. What the user above was trying to say is that, typically, &#916;H is given in J/mol but that's not always case (like the above problem for example). Check the units and you'll be fine.

#6
Last edited: Aug 6, 2011
7. OP

pandalove89 7+ Year Member

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Ahhhh! I get it. Thank you.

But can specific heat (c) be in J/mol as well?

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