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Offline helpmeimnoob

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Chem Lab Error Problem
« on: January 06, 2008, 06:58:37 PM »
I did a chem lab in high school, and one of the questions asked me to find the molar heat of fusion of ice. I know it is suppose to be 6.01 kJ/ mole, but I am suppose to have an answer close to that (because of the lab's error analysis). How would I find the molar heat of fusion of ice with this data?

Mass of Styrofoam cup and lid
7.74g
Mass of cup, hot water, and lid
104.5g
Mass of hot water
96.76g
Mass of ice used
17.66g
Initial temperature of water
50.5 Degrees C
Final water temperature
29.4 Degrees C

Offline Alpha-Omega

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Re: Chem Lab Error Problem
« Reply #1 on: January 06, 2008, 08:10:06 PM »
OK I am not sure you have really done anything with this data.  But here is the procedure you have to follow to traet your data:

Calculations:

1. Calculate the mass of water in the calorimeter in kilograms.

2. Calculate the mass of ice added to the calorimeter in kilograms.

3. Calculate the moles of ice that was added (mass of ice in grams ÷ molar mass of water) = n.

4. Calculate the temperature change in the water in the calorimeter (temperature before - temperature after).

5. Calculate the temperature change that happened to the ice after it melted (this is the change from 0 oC to the final temperature of the ice water mixture).

6. The heat that the water in the calorimeter lost was absorbed by the ice in first melting, and then later in the ice warming up to the final temperature of the water in the calorimeter.


Heat lost by the calorimeter Q calorimeter = m * c *t
Q = heat in KJ

m = mass of water in calorimeter in Kilograms (#1 above)

c = specific heat capacity of water (4.19 J/g * oC)

 t = temperature change in calorimeter.(#4 above)

Heat gained by the ice in melting is Q melting of ice =  H * n
Q = heat in KJ

H = molar enthalpy of fusion of ice

n = moles of ice (#3 above)

Heat gained by the ice as it warms up to the final temperature in the calorimeter is Q ice water = m* c *t
Q = heat in KJ

m = mass of melted ice (same as the mass of the ice) (#2 above)
 
c = specific heat capacity of melted ice (same as water = 4.19 J/g * oC)

t = temperature change that occurred in the melted ice (#5 above )

Combined the formula is as follows:

Heat change in calorimeter  = Heat needed to melt ice + Heat absorbed as ice warmed up

m * c * t                         =         H * n                  +                m * c * t

7. Plug in values from above into this formula and calculate the molar heat of fusion of ice (  H )

Since you mention ERROR ANALYSIS:  Procedure is as follows-


The accepted value for the molar heat of fusion of ice is 6.03 KJ/mol.

Calculate a % error as follows.
% error = |(measured value - accepted value (6.03 KJ))| ÷ accepted value
 
The bars indicate that you take the positive value of the difference between measured and accepted values.








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