Practical Skills - Errors

Question 1

What is the main problem of making measurements with apparatus?

Answer 1

There is always a small uncertainty in measurement due to the apparatus itself.

Question 2

What do you call uncertainties when measuring with apparatus?

Answer 2

apparatus error

Question 3

Can apparatus error be avoided?

Answer 3

No, only reduced

Question 4

How do you reduce apparatus error?

Answer 4

Use the highest resolution equipment available.

For example, when measuring out 25 cm³ of a solution, a pipette is much offers higher resolution than a measuring cylinder

Question 5

How do you calculate apparatus error?

Answer 5

Question 6

What is total apparatus error?

Answer 6

Sum of all apparatus errors in the experiment

Question 7

A pupil doing an experiment measures out 1.245 g of a base using a balance, then makes it up to 250 cm³ of solution in a volumetric flask. They then pipetted 25 cm³ of that solution into a conical flask, and then found that it reacted with 23.30 cm³ of acid in a titration using a burette.

The balance measures to (± 0.001 g).

What is the apparatus error?

Answer 7

100 x (0.001/1.245) = 0.08%

Question 8

A pupil doing an experiment measures out 1.245 g of a base using a balance, then makes it up to 250 cm³ of solution in a volumetric flask. They then pipetted 25 cm³ of that solution into a conical flask, and then found that it reacted with 23.30 cm³ of acid in a titration using a burette.

The pipette measures to (± 0.1 cm³ )

What is the apparatus error?

Answer 8

100 x (0.1/25) = 0.40%

Question 9

A pupil doing an experiment measures out 1.245 g of a base using a balance, then makes it up to 250 cm³ of solution in a volumetric flask. They then pipetted 25 cm³ of that solution into a conical flask, and then found that it reacted with 23.30 cm³ of acid in a titration using a burette.

The volumetric flask measures to (± 0.1 cm³ )

What is the apparatus error?

Answer 9

100 x (0.1/250) = 0.04%

Question 10

In an experiment the equoiment has the following apparatus errors.

Balance 0.08%

Pipette 0.40%

Volumetric flask 0.04%

Burette 0.64%

What is the total apparatus error?

Answer 10

0.08 + 0.40 + 0.04 + 0.64 = 1.16%

Question 11

When we design experiments why are we careful with the equipment we use?

Answer 11

So that apparatus errors are minimised. E.g. by using, a piece of apparatus that is a similar scale to what we measure (so don’t use a 250cm³ measuring cylinder to measure 5cm³)

Question 12

What is experimental error?

Answer 12

Errors caused by less than perfect technique

Question 13

How do you calculate experimental error?

Answer 13

Question 14

Using experimental error, how do you know you have an accurate result?

Answer 14

If experimental error is smaller than apparatus error, then you have an accurate result. However, if experimental error is larger than apparatus error, then the result is inaccurate.

Question 15

In a titration the concentration of a solution is found to be 0.0995 mol dm^-3. The real value is 0.101 mol dm^-3.

What is the experimental error?

Answer 15

Question 16

After completing an experiment a student calculates the experimental error to be 1.49% and the apparatus error to be 1.16%. What can you say about the results?

Answer 16

They are inaccurate. If experimental error is smaller than apparatus error, then you have an accurate result. However, if experimental error is larger than apparatus error, then the result is inaccurate.