 ## General Chemistry Lab Notes

Printable files require a PDF reader for viewing. If you do not have one get Adobe Reader. ### Missing a Lab?

It is best not to miss any labs because a worksheet must be submitted and graded each day.

If you do miss a lab, you have the option to attend another section where the lab that you missed (or will miss) is being carried out. For a list of these sections, check the course listing in the college homepage.

Once you find a lab section, you can ask the instructor if it is OK to stay in the room and do the experiment.

The instructor reserves the right to deny your request. This will likely happen if the room is full.

If the experiment you need to do was done in a previous week, then all of the above mentioned procedure is useless and you will not be able to recover the points for the lab you missed.

### Reporting results with error bars

If you are having problems with error bars, download the document about uncertainties in measurement reporting. This document was meant for a Physical Chemistry class but it is all about standard statistics.
Uncertainties in measurement reporting
With sample calculations [PDF] 2.73 mb

### Preparing a standard Sodium Hydroxide solution

The stock solution is usually 2 or 3 mol/L. Let's assume it is 3 mol/L and you want to prepare 400 mL of a Sodium Hydroxide solution with approximate concentration 0.1 mol/L.

```Use the standard formula CiVi = CfVf
Ci, Vi = Initial concentration and volume
Cf, Vf = Final concentration and volume
(3 mol/L) Vi = (0.1 mol/L) 400 X 10-3 L
(0.1 mol/L) 400 X 10-3 L
Vi = ----------------------------------
(3 mol/L)
Vi = 13.3 X 10-3 L
Vi = 13.3 mL
```
Thus you will need 13.3 mL of 3 mol/L stock solution then add DI water to make 400 mL of solution: (400 - 13.3) = 386.7 mL.

However, the volume does not need to be exact because you need to standardize the solution. How do we calculate the exact concentration of dilute sodium hydroxide from the titration data?

Say you prepared a standard KHP solution by dissolving 4.5 g of KHP in 250 mL of water. Note the KHP does not have to be exactly 4 or 5 g, but the amount must be known exactly.
Now suppose you take 35.00 mL out of the 250 mL KHP solution and titrate with the dilute Sodium Hydroxide solution. The equivalence point is reached after you add 32.50 mL of the dilute Sodium Hydroxide solution into the 35.00 mL of KHP solution.

```                             4.5 g
Mol of KHP in 4.5 g = --------------------- = 0.022035 mol
204.22 g/mol)
35
KHP in 35.00 mL = ------------- X 0.022035 mol  = 0.0030849 mol
250
Recall that mol of KHP = mol of NaOH
KHP + NAOH --> KNaP + H2O
0.0034049 mol
Molarity of NaOH = -------------------------- = 0.09492 mol/L
32.50 X 10-3 L
```

Thus your standard Sodium Hydroxide solution has a concentration of 0.09492 mol/L, which is very close to the 0.1 mol/L we were aiming to when making the dilution from the 3 mol/L stock solution.

### The empirical formula of a hydrate

When 5.061 g of MgSO4•xH2O is heated to evaporate all the water, 2.472 g of MgSO4 is recovered. Find the value of x.

```g of MgSO4 = 2.472 g
g of H2O = 5.061 g - 2.472 g = 2.589
Use MgSO4 = 120.4 g/mol
5.061 g of MgSO4
-------------------- = 0.02053 mol MgSO4
120.4 g/mol
Use H2 = 18.02 g/mol
2.589 g of H2O
-------------------- = 0.1437 mol H2O
18.02 g/mol
Ratio H2O:MgSO4
0.1437 mol H2O
----------------------- = 7.000
0.02053 mol MgSO4
```
Thus there are seven (7) water molecules for every MgSO4 unit.

Empirical formula: MgSO4•7H2O

### Balancing RedOx reactions with the ion-electron method

Redox reactions are commonly run in acidic solution, in which case the reaction equations often include H2O(l), OH-(aq) and H+(aq) but you might not know whether the H2O(l), OH-(aq) and H+(aq) are reactants or products.

Ion-Electron method Outline

•  Divide equation into half-reaction
•  Balance atoms other than H and O
•  Balance O by adding H2O
•  Balance H by adding H+
•  Balance net charge by adding e-
•  Make e- gain equal to e- loss and add half reactions
•  Add half reactions and cancel anything that is the same on both sides

•  Add to both sides of the equation the same number of OH- as there are H+
•  Combine OH- and H+ to form H2O
•  Cancel any H2O that you can

For example, the following reaction occurs in acidic solution:
IO3- + Re → ReO4- + I-

•  Divide equation into half-reaction
IO3- → I-
Re → ReO4-
•  Balance atoms other than H and O (no atoms to balance here)
IO3- → I-
Re → ReO4-
•  Balance O by adding H2O
IO3- → I- + 3H2O
4H2O + Re → ReO4-
•  Balance H by adding H+
6H+ + IO3- → I- + 3H2O
4H2O + Re → ReO4- + 8H+
•  Balance net charge by adding e-
6e- + 6H+ + IO3- → I- + 3H2O
4H2O + Re → ReO4- + 8H+ + 7e-
•  Make e- gain equal to e- loss and add half reactions
{6e- + 6H+ + IO3- → I- + 3H2O} X7
{4H2O + Re → ReO4- + 8H+ + 7e-} X6

42e- + 42H+ + 7IO3- → 7I- + 21H2O
24H2O + 6Re → 6ReO4- + 48H+ + 42e-
•  Add half reactions and cancel anything that is the same on both sides
42e- + 42H+ + 7IO3- + 24H2O + 6Re →
7I- + 21H2O + 6ReO4- + 48H+ + 42e-

7IO3- + 3H2O + 6Re → 7I- + 6ReO4- + 6H+

•  Add to both sides of the equation the same number of OH- as there are H+
6OH- + 7IO3- + 3H2O + 6Re → 7I- + 6ReO4- + 6H+ + 6OH-

•  Combine OH- and H+ to form H2O
6OH- + 7IO3- + 3H2O + 6Re → 7I- + 6ReO4- + 6H2O

•  Cancel any H2O that you can
6OH- + 7IO3- + 6Re → 7I- + 6ReO4- + 3H2O

RedOx Reactions
With sample problems [PDF] 2.53 mb

### Metal Recovery from RedOx reaction: aluminum metal + copper chloride-> aluminum chloride + copper metal

These are the masses of copper recovered when different masses of aluminum foil are reacted with 10 mL of the copper chloride stock solution. The aluminum is the limiting reactant and the mass is kept lower than 60 milligrams.

``` -- Mass of Cu obtained in last Exp.
mg of Al ... mg of Cu
14 ............... 52
18 ............... 67
20 ............... 70
22 ............... 80
25 ............... 87
26 ............... 94
29 ............... 100
30 ............... 105
32 ............... 118
35 ............... 129
36 ............... 125
38 ............... 140
39 ............... 135
40 ............... 140
41 ............... 145
43 ............... 159
44 ............... 155
46 ............... 165
48 ............... 170
53 ............... 186
55 ............... 195
```

### Lab Files From Last Year

Exp #10

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