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### Topic: Need help with calculations for lab  (Read 143 times)

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#### candies

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• Mole Snacks: +0/-0 ##### Need help with calculations for lab
« on: November 17, 2020, 07:49:21 PM »
I am doing this lab (thanks online learning) and I am getting incorrect calculations. I think I have them right so far but am not sure.

Objectives: To prepare several solutions and determine the concentrations of each; to perform some dilutions and become determine the concentration of the new solutions.

Volume percent:

DO: Using the 50-mL graduated cylinder, measure out between 10 and 15 mL of green water and record to the correct significant figures.  Add water to between 40 and 50 mL; record the volume.  Rinse the graduated cylinder when done.

1.      Volume of green water                  12.5 mL

2.      Total volume of water after addition            47.5 mL

3.      Volume percent of green water (show calculation)    (12.5/47.5)*100=26.3%

Sodium chloride solution:

DO:  Use half of a spoonful of blue salt and a small beaker:

4.   Mass of beaker                      29.938 g

5.   Mass of beaker + NaCl                     33.842 g

6.   Mass of NaCl                        3.904g

DO: Find the mass of an empty graduated cylinder, then add about 40 mL of water to the graduated cylinder.  Record the volume of the water, as well as the mass of the graduated cylinder with water.

7.   Mass of empty graduated cylinder                126.42 g

8.   Volume of water                     40.0 mL

9.   Mass of graduated cylinder + water               165.44 g

10.   Mass of water                        39.02g

DO: Add the water from the graduated cylinder into the beaker with the salt, and stir.  Pour the solution into a 100-mL graduated cylinder and record the total volume (big assumption here: we are assuming every drop is transferred back into the graduated cylinder).  If the volume is less than the volume of water (in #8), use the volume of water from #8.  **DO NOT dump your solutions!! **

11.   TOTAL Volume of solution (blue salt water)            41.5 mL
Use the following values from the previous page to perform the following calculations for this solution.  Show all calculations!

mass NaCl (#6) 3.904g ,  mass water (#10) 39.02g ,  Total volume soln (#11) 41.5mL

12.   What is the mass percent (m/m) of this solution?         _9.407%_____
(3.904/41.5)*100

13.   What is the ppm of the NaCl in this solution?            __94072 ppm___
1% = 10,000 ppm

14.   What is the mass/volume percent (m/v) of this solution?      __9.096%_______

15.   What is the concentration of this solution in grams/liter?      _1.610g/L________

16.   What is the molar mass of NaCl?               58.4428 g/mol

17.   How many moles of NaCl are in this solution?            __0.067mol____

18.   What is the molarity, M (moles/L) of the solution?         __1.610M_____

Now DILUTE the solution!

DO: Add ~45 mL of water to the blue solution; record to appropriate significant figures

19.   Volume of water added                     45.0 mL
20.   What is the new volume of the solution?            ___86.5mL____
(initial volume (#11) _41.5__ mL + new volume __45.0__mL)

21.   How many moles of NaCl are in the solution (see # 17)?      ___0.067mol____

22.   Calculate the molarity of the new solution.            ___0.77M_______

DO:  NOW: Using a 10-mL graduated cylinder, measure out an ~8 mL aliquot (Record the volume to the correct significant figures) of your blue salt solution and put it into your (empty) 50-mL graduated cylinder.

23.   Volume of aliquot of salt solution,                               8.0 mL, = _0.008_ L
(watch sig figs!)

24.   Moles of NaCl in aliquot of salt solution (M1 x V1)
M1  [from #22] = 0.77___ moles/L,

V1 [from #23] = __0.008__L
Show the Calculation of moles:   (M1 x V1) =               __0.00616moles__

NOW Dilute the solution again!!

DO:  Add an additional 20 or 25 mL of water (to the 8 mL aliquot) and record the new volume.

25.   New volume of solution.             38.5 mL, =  0.0385___ L (Sig figs!)

26.   Calculate the new molarity of solution.               -__0.16M_________
(use moles calculated in #24)
NOW clean up!!

Now do this same calculation using the dilution formula, M1V1  =  M2V2 , where:
M1 = initial molarity (#22) = 0.77M_   M2 = final molarity (calculated)

V1 = initial volume (#23)    = 8ml___   V2 = final volume (#25) = 38.5_____

Note: you may leave the volume in mL since the conversion to L cancels.

M2 = 0.77*8=38.5*?

4.312/38.5 = 0.16M

(You should get the same molarity you calculated in #26.)
« Last Edit: November 17, 2020, 10:26:11 PM by candies »

#### Borek ##### Re: Need help with calculations for lab
« Reply #1 on: November 18, 2020, 03:22:23 AM »
12.   What is the mass percent (m/m) of this solution?         _9.407%_____
(3.904/41.5)*100