Calculating Mass of Water in NH4Cl Solution After Temp Change

  • Thread starter kuahji
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In summary, the conversation discusses a problem involving finding the mass of water in a solution that decreases in temperature after adding 2.00 g of NH4Cl. The enthalpy of solution and specific heat of the solution are given, and the equation used is change in temp = q/(specific heat * mass of solution). The individual trying to solve the problem initially gets a result of 700g, but after making a correction for units, the correct answer of 24.5 g is found.
  • #1
kuahji
394
2
Ok, where am I going wrong on the following problem
What mass of water is in a solution that decreases in temperature from 25.0 to 20.0 when 2.00 g of NH4Cl are added? (The enthalpy of solution is 14.8 kJ/mol, and the specific heat of the solution is assumed to be 4.18 J/g·K.)

I set the problem up where the change in temp = q/(specific heat * mass of solution)

5K = 14800J/(4.18 J/g*K * (2g+xg)) Then I just solved for x & I keep getting around 700g. But the answer is suppose to be 24.5 g. Any ideas where my logic is breaking down?
 
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  • #2
place a negative sign in front of 5 K -___-;;

kindly note that change in temperature = final - initial...

do remember ok? :)
 
  • #3
um... ok, that doesn't really change anything... if you look at my equation I already changed q to a postive 14800, whereas it would have been negative along with the change in temp, its a step I skipped showing. Which still doesn't help me, I'm still getting a little over 700g, & the answer is suppose to be 24.5. If I just change the 5 to a negative, I get negative grams :(. Again, it was already compensated for.
 
  • #4
How many moles?
 
  • #5
Oh hehheh, bystander spotted it ;p...

Kuahji, it should be 148000 J/mol rather than 148000 J

do a conservation of units, you should be able to work out the answer...
 
  • #6
Yes, that work... for whatever reason I kept forgetting J/mol. Thanks again.
 

Related to Calculating Mass of Water in NH4Cl Solution After Temp Change

1. How do you calculate the mass of water in a solution after a temperature change?

The mass of water in a solution can be calculated by first finding the mass of the entire solution, then subtracting the mass of the solute (in this case, NH4Cl) from the total mass. The remaining mass will be the mass of water in the solution.

2. What is the formula for calculating the mass of water in a solution?

The formula for calculating the mass of water in a solution is: Mass of water = Total mass of solution - Mass of solute.

3. How does temperature affect the mass of a solution?

Temperature can affect the mass of a solution because it can cause changes in the volume of the solution. As temperature increases, the volume of a solution may also increase, leading to a decrease in mass. This is due to the expansion of water molecules as they gain energy.

4. Will the mass of water in a solution change if the temperature changes?

Yes, the mass of water in a solution may change if the temperature changes. This is because temperature can affect the volume of the solution, which in turn affects the mass. However, the mass of the solute will remain constant.

5. How can you accurately measure the mass of a solution after a temperature change?

To accurately measure the mass of a solution after a temperature change, it is important to use a calibrated balance or scale. This will ensure that the measurements are precise and reliable. It is also important to record the temperature of the solution at the time of measurement, as this can affect the mass calculation.

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