Techniques Lab

Lab Notebook• See pages 10-11

• Prelab-notebook: (Required for each lab; 2 marks) o Title - 0.25 mark

o Purpose - 0.25 mark (2-3 sentences briefly stating main concepts, reactions and

techniques)

o Chemical Hazards – 1.5 mark (Toxicological properties, preventative measures, first aid

measures)

• Write in pen

• Write on the original page and insert divider under

copy sheet before wiring

• Put your name on the front and leave room for a Table

of Contents

Determinate and Indeterminate Errors

• Determinate errors: systematic, experimentalo i.e. inaccurate equipment, impure reagents

• Indeterminate errors: operator, randomo i.e. Losses from transfers, differences in judgement

Precision versus Accuracy

http://celebrating200years.noaa.gov/magazine/tct/accuracy_vs_precision.html 2010-09-14

Precision:

agreement

between various

measurements

Accuracy:

agreement

between the

average result and

the actual value

Uncertainty• There is always some uncertainty in measurement

• Equipment in this lab has typical uncertaintyo See page 43

Instrument Typical Uncertainty

Graduated cylinder, 10 mL ± 0.1 mL

Graduated cylinder, 100 mL ± 0.2 mL

Pipette, 5 mL ± 0.01 mL

Pipette, 10 mL ± 0.02 mL

Pipette, 25 mL ± 0.03 mL

Balance: Electronic top loader ± 0.01 g

Balance: analytical, MettlerAE160/AE200

± 0.0001 g

Significant Figures• Significant figures help to identify the precision of a

measuremento Zeros to denote position of decimal are not sig figs

• Example: using a 10 mL graduated cylinder, measured 6.2 mLo According to the typical uncertainty in equipment, the data would be

recorded as 6.2 ± 0.1 mL

o 2 sig figs in the measurement

o 6 is certain, 2 is uncertain

Number of sig figs

0.0301 g 3

4.290 hours 4

1.030 mg 4

Number of sig figs

0.001030 g 4

5000 miles 1 (but ambiguous)

5.000 x103

miles4

Significant Figures in Calculations

• Addition and Subtraction:o Significant figures should be reported to the precision of the term of lowest

precision

24.35

- 9.575

+11.2

+0.46

26.435 -> 26.4 (3 significant figures)

Note: Keep the least decimal places

o Perform the calculation and then round off the digits

Significant Figures in Calculations

• Multiplication and Divisiono Significant figures are reported to the precision of the term of lowest

relative precision

•���.���×.�

�.��= 5.648 × 10� → 5.6 × 10� (2 sig figs)

Note: Keep the least sig.fig.

Significant Figures in Calculations

• Logarithms and Antilogarithmso The number of sig fig. in the number = The number of decimal places in

the logarithm

o For example: log 1.21 = 0.083

3 sig. figs - 3 decimal places

o In pH calculations

if [H+] = 0.047M, pH = -log 0.047 (2 sig. figs)

= -log 4.7x10-2

= 2.00 - log 4.7

= 2.00 - 0.67(2)

= 1.33 (2 decimal places)

Significant Figures in Calculations

• Multistep calculations:o Calculate intermediate results beyond the necessary precision and round

off at the end

• Rounding:o If the figure next to the last retained digit is 5 or more round up

o If the If the figure next to the last retained digit is lee than 5 round down

o 4.6349 4.635 (to 4 sig figs)

o 4.6349 4.63 (to 3 sig figs)

Uncertainty in Calculations

• Addition and Subtractiono Uncertainty is the sum of the uncertainties of each component

measurement

• Length measuremento Between 1.38 ± 0.02 cm and 13.85 ± 0.02 cm

13.85 ± 0.02

- 1.38 ± 0.02

12.47 ± 0.04 cm

• Uncertainty of the Averageo Three trials of a length measurement:(1)18.50 ± 0.04 cm; (2)18.40 ± 0.04

cm; (3)18.60 ± 0.04 cm

o Average : (18.50 +18.40 +18.60 )/3 = 18.50

(0.04+0.04+0.04)/3 = 0.04

Average =18.50 ± 0.04 cm

Uncertainty in Calculations

• Multiplication and Divisiono The uncertainty is the sum of the percentage uncertainties of each of the

component measurements

6.2 ±0.2 cm x 7.6 ±0.2 cm

6.2 cm x 7.6 cm = 47.12 -> 47cm (2 sig figs)

(0.2/6.2)*100 + (0.2/7.6)*100 =

3.226... + 2.631… = 5.857 % = 6% (1 sig fig)

(47.12 cm / 100%) * 6 % = 2.82 cm -> 3 cm (1sig fig)

Answer: 47 ± 3 cm

Evaluation of Precision• Relative spread:

• Equation: ���������������������� ���!"#�!

$%��$&�%$#"�× 1000 =

(24.39 − 24.20)

24.29× 1000 = 8--�

Sample Volume (mL)

1 24.39

2 24.20

3 24.28

Average 24.29

Evaluation of AccuracySample Volume (mL)

1 24.39

2 24.20

3 24.28

Average 24.29

Accept Value 25.00

• % error

% error = (Average – Accept Value)/Accept Value

= (24.29 – 25.00)/25.00

= - 2.840%

Note: % error with a negative number means the experimental result is lower than the accept value.

Pipetting• See also pages 29-31

• Don’t blow out last drop, simply touch the last drop

to the side

Measuring Length• See also pages 38 and 45

• Uncertainty depends on:o the graduation of the scale

o Thickness of what is being measured

o Skill of the measurer

o Use the smallest graduation to estimate the uncertainty

Balances• See also pages 33-34

• Do not insert tools into reagent bottles

• Do not put chemicals back into reagent bottles

• Keep the balances clean

Techniques lab • Part A

o Practice with balance and be sure to record the correct sig figs and

uncertainties.

o Use the analytical balance for the copper and the top loading balance

for the water

• Part Co Use the table but measure only the length and width of the student card

• Work in Pairs

• Get your notebook signed by a TA

For next lab • Do your WHMIS training, buy a labcoat, goggles,

hardbound notebook and pay your lab fee if you

have not already done so

• read and prepare for the next lab (according to

the schedule)o Prelab on notebook (Title, Purpose, Chemical Hazards)

• Finish for a quiz and pre-lab calculations on cuLearn

regarding the next labo Generally questions concerning the prelab, theory, procedure, equations,

calculations, safety steps, etc