Chapter 1.2 Part B: Chemical Apparatus Grade 10 (Level L)

 

1.2.8 Using a measuring cylinder

A measuring cylinder is used to measure volumes of liquids where great accuracy is not important.

Measuring cylinders come in various sizes and it is important to use the most appropriate cylinder for the volume to be measured e.g. if 10.0cm3 ± 0.5 ml of liquid is required, then either a 10ml or 25ml cylinder should be used and not a 250ml cylinder.

Q

 If we want to measure 30 ml of water using a measuring cylinder, which of the following measuring cylinders would be most suitable?

• 25 ml measuring cylinder
• 50 ml measuring cylinder
• 100 ml measuring cylinder
• 250 ml measuring cylinder

 correct answer

50 ml measuring cylinder

1.2.8 Using a measuring cylinder – Cont.

When reading a measuring cylinder you must:

a) stand the cylinder on the bench, do not hold it in your hand.

b) bend down so that your eye is on the same level as the surface of the liquid when you read the scale.

c) measure to the bottom of the curved surface of the liquid. This curved surface is called a meniscus.

For very accurate measurements of volume (with an uncertainty between ± 0.05 cm3 and 0.1cm3), pipettes and burettes must be used (explained later).

1.2.9 Filtration

Filtration is the process of separating a liquid from an insoluble solid (i.e. does not dissolve in the liquid). A beaker, filter stand, filter funnel and a piece of filter paper are required.

It is important to fold the filter paper properly and this is done as follows:

a) Fold the filter paper in half and then into quarters.

b) Open the paper out to make a cone with a thickness of three on one side and one on the other.

The filter paper now fits neatly into the filter funnel. Wet the filter paper before starting the filtration and always pour liquid onto the three-thickness side and never allow liquid to come within 1cm of the top of the paper.

Never poke a wet filter paper with your finger, a spatula or a glass rod as this may tear the paper and you will have to start again.

How do you fold a filter paper to fit it in a funnel?

Fold the filter paper in half and then into quarters.

Open the paper out to make a cone with equal thickness on each side.

Open the paper out to make a cone with a thickness of three on one side and one on the other.

Wet the paper with distilled water then fold it into half.

Label the figure below.

The liquid, which comes through the filter paper, is known as the filtrate and the solid, is left behind on the paper, is called the residue.

Fill in the blanks.

  ….. is the process of separating a liquid from an insoluble solid.

Fill in the blanks.

The …  is the liquid which comes through the filter paper.

Fill in the blanks.

The .... is the solid left behind on the paper.

1.2.10 Measuring mass

The Electric Balance

In earlier courses you have learned how to use a beam balance to measure to the nearest 0.1 g. Now, you have to be able to measure to the nearest 0.01 g. To do this, you will need to use an electric balance. Your teacher will demonstrate its use.

Weighing by Difference

When measuring a mass, either using the ‘dial-O-gram’ or the electronic scale, it is always more accurate to weigh by difference whenever possible.

Suppose you would like to measure about of 1g of a solid X. Do the following:

a) Weigh accurately and record the mass of a weighing bottle (m1).
b) Add approximately 1g of a solid to the bottle and accurately record the mass (m2).
c) The accurate mass of the solid is obtained by subtraction (m2 - m1).

This method reduces the effect of any mechanical error that may be present in the balance.

A pipette is designed to measure a specified fixed volume of liquid when correctly filled, usually to the nearest 0.05 cm3. In other words you can only select one of a specified number of volumes, not in between. The common capacities of pipettes are 5 ml, 10 ml, 25 ml and 50 ml.

Never use your mouth to suck liquid into a pipette. Instead, use a pipette filler. A pipette filler has three valves, marked either A, S and E or 1, 2 and 3.

a) Attach the filler to the pipette.

b) Squeeze valve A (for air out) (valve 1) and at the same time squeeze the bulb of the filler. This drives the air out of the filler.

c) Place the pipette in the liquid to be used.

d) Squeeze valve S (for suck) (valve 2) and the liquid is sucked into the pipette. Continue until the liquid is a few centimeters above the calibration mark.

e) Move your eyes to the same level as the calibration mark and carefully squeeze valve E (for empty) (valve 3) to empty some liquid until the bottom of the liquid meniscus just touches the calibration mark. (If you overshoot then you must go back and repeat steps ‘d’ and ‘e’)

f ) Remove the filler from the pipette and allow the liquid to run out into a clean beaker or flask.

g) After the liquid has run out, touch the bottom of the pipette against the side of the beaker or flask. A little liquid should still be left in the tip of the pipette. Do not blow out this liquid into the beaker or flask as the measurement requires this liquid to be left over.

Order the steps involved in measuring and discharging 50 cm3 of a solution using a pipette.

Squeeze valve S to suck the liquid into the pipette until the liquid is a few centimeters above the calibration mark.

Squeeze valve A and at the same time squeeze the bulb of the filler.

Carefully squeeze valve E to empty some liquid until the bottom of the liquid meniscus just touches the calibration mark.

Place the pipette in the liquid to be used.

Remove the filler from the pipette and allow the liquid to run out into a clean beaker or flask.

Attach the filler to a 50 mL pipette.

.

After the liquid has run out, touch the bottom of the pipette against the side of the beaker or flask. A little liquid should still be left in the tip of the pipette.

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