Accounting for mass in phase changes

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Abigail Urbina 1K
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Accounting for mass in phase changes

Postby Abigail Urbina 1K » Wed Jan 17, 2018 3:39 pm

For questions such as homework problem 8.41, where you are asked to calculate enthalpy changes where phase changes are involved, how do you know when to account for the molecular weight of a substance? In other words, if you are given the mass of a substance, how exactly do you know when to divide that given mass by a molecular weight?

For example, 8.41 asks: A 50.0-g ice cube at 0.0 C is added to a glass containing 400.0 g of water at 45.0 C. What is the final temperature of the system (see Tables 8.2 and 8.3)? Assume that no heat is lost to the surroundings.

In the solution manual, the 50.0 g is divided by the molecular weight of water which is 18.02 g/mol. Do you only include molecular weight in your calculations when there is a phase change you are calculating the enthalpy for? Because once the ice is changed into liquid water the calculations for the heat of water only take into account the specific heat capacity of water.

Naveed Zaman 1C
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Re: Accounting for mass in phase changes

Postby Naveed Zaman 1C » Wed Jan 17, 2018 4:17 pm

You can always write out units for each of the numbers given to you if it helps. Here, we are given the mass of the substance in grams, but the heat capacities given are MOLAR heat capacities. Therefore, we have to convert grams to moles to use it with the molar heat capacity. However, you can also just use the specific heat capacity and keep the given mass in grams, and you should get the same result.

Veronica Rasmusen 2B
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Joined: Sat Jul 22, 2017 3:01 am

Re: Accounting for mass in phase changes

Postby Veronica Rasmusen 2B » Mon Jan 22, 2018 11:41 pm

It depends what units your constants are in. If the heat capacity given uses moles, then you would have to convert grams to moles so the units cancel. If given specific heat capacity using grams you would not need to convert grams to moles.

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