MEASUREMENTS ACCURACY &PRECISION

For liquids in thin tubes we read thebottom of the meniscuswith our eye at this level

•What is the reading on this graduated cylinder?•Where exactly do you take the reading?•What is the uncertainty?A 40.5 mlB 40.5 ± 0.5mlC 45.0 mlD 45.0 ± 0.5mlE 45.0± 1.0ml

•What is the volume reading on this burette?•Clearly between 30 and 31 ml.• The miniscus looks like 30.05 ± 0.05• Since 0.05 is half the smallest division

For uncertaintyFirst look on the Apparatus forTolerance, in thisCase ± 0.750ml.If this is not Available, take HALF the smallestDivision (± 0.5ml)

How accurate are the measurements on the side of aBeaker?

They are approximate

What about conical flask volumes?Again these are only approximate

Maybe ± 10 to20%

Your turn?What is the reading

and uncertainty here ?

What is this instrument

Called ?See the tolerance

(uncertainty) clearlylabelled

This is a volumetric flask.One of the more accurate pieces we havefor volumes of solutions.

Notice the toleranceis for 20oC so donot put warm hands around the bowl.

The most accurate piece of apparatus in a chemistry labis the electronic balance.

Our best balance is ± 0.002g for each reading

This is the less accurate electronic balance with an uncertainty of ±0.01g. Care need to be taken with the mode to ensure it is on the grams scale and not ozs

What we do with uncertainties

• Mass of beaker + copper(II)oxide = 26.325±0.002g

• Mass of beaker after transfer = 22.234±0.002g• Mass of copper(II)oxide = 4.091±0,004g

• Notice how we add the uncertainties• We do this whether we are subtracting or adding

What do we do if x or divide?

• Consider a density calculation where• density= mass/volume• Mass is1.141±0.002g 0.002 x 100% = 0.2%

1.141

Volume is 1.55±0.05ml 0.05 x 100% =3.2% 1.55

Density = 1.141±3.4% 0.736±0.025g/ml 1.55

We change to %, add %, then convert back to number

Besides Quantitative measurements we also need toRecord qualitative, including colour changes.

Here is an example of an indicator changing fromIt’s acid colour, via orange, to it’s alkaline colour, yellow

This , in my opinion, is a clearer colour change for the sameReaction. Do not forget the colour before and after a reaction.

Comparing different temperature scales

Temperature scales

We only use Celsius and Kelvin and Kelvin the correct SI Unit to use.

Measuring liquid volumes using a buret (burette). It is very important to get your eye level with the meniscus to avoid errors. Also remember with burettes you are running liquid out so the scale increases going down the buret

Measurements

Accuracy and precision

Neither accurate or precise

Precise but not accurate

Both accurate and precise

Accuracy & Precision

HOW CLOSE IS CLOSE?

It’s not a perfect world, so there are no perfect answers.

THE ANSWER IS:

• How close is close, depends on your acceptable percentage of error.

• Acceptable percentage of error depends on– The precision of your instrument– Whatever your teacher says is acceptable.

LET’S LEARN SOME TERMINOLOGY

• Experimental value – the value obtained from doing an

experiment• Actual value or theoretical value

– the value found in literature as the accepted or standard value

CALCULATING % ERROR

actual value experimental value x 100% actual value

SAMPLE CALCULATION

In a mass/volume experiment to determine the density of gold, a student calculated the density to be 18.75 g/mL. The actual value for the density of gold is 19.32 g/mL. What is the percent error?

SAMPLE CALCULATION

In a mass/volume experiment to determine the density of gold, a student calculated the density to be 18.75 g/mL. The actual value for the density of gold is 19.32 g/mL. What is the percent error?

% ERROR CALCULATION

| 19.32 – 18.75| x 100% = 3.0 % 19.32

WHAT IS AN ACCEPTABLE % ERROR?

• Is 3.0% error a good or a bad result?• That depends upon

– The precision of the instruments used– And ultimately the expectation of the teacher.

• In this case, 3.0% is very good because of the instruments available in a typical school lab.