Concentration equationsconcentration (g/dm3) = mass (g)/volume (dm3)

NOTE: 1dm3 = 1000cm3

Conversions

cm3 to dm3: vol/1000 e.g. 25.0cm3 = 0.025dm3

dm3 to cm3: volx1000 e.g. 0.125dm3 = 125cm3

mass (g) = concentration (g/dm3) x volume (dm3)

volume (dm3) = mass (g)/concentration (g/dm3)

Calculate the concentration (in g/dm3) of:

1. 3g Na2CO3 in 250cm3

2. 0.5g MgCl2 in 25cm3

Calculate the mass of solute in:

1. 100cm3 2.5g/dm3 CuSO4 (aq)

2. 25cm3 0.1g/dm3 CH3COOH (aq)

Calculate the volume of solution needed to provide the mass of solute:

1. 4g KBr from a 16g/dm3 solution

= mass/volume (dm3) =3/(250÷1000) = 12g/dm3

= 0.5/(25÷1000) = 20g/dm3

=concentration x volume = (100÷1000) x 2.5 = 0.25g

= (25÷1000) x 0.1 = 0.0025g

= mass/concentration = 4/16 = 0.25dm3 = 250cm3

Making up a standard solution• I am going to make 250 cm3 of 5 g/dm3 sodium

carbonate1. Mass Na2CO3 = concentration (g/dm3) x volume (in dm3)

2. Mass Na2CO3 = 5 x (250/1000) = 1.325 g

3. Dissolve in about 100 cm3 distilled water

4. Transfer to 250.0 cm3 volumetric flask

5. Add rinsings to flask – ensures all of the Na2CO3 weighed is in the flask

6. Make up to mark

7. Reading to bottom of the meniscus

• Using a safety filler pipette a 25 cm3 sample of vinegar (already diluted ten times) into a flask.

• Add 5 drops of phenolphthalein indicator.• Read your burette and record the reading. Carry out a rough

titration. Run sodium hydroxide solution into the flask until the indicator changes from colourless to permanent pink colour. Reread the burette.

• Refill the burette and record the reading. Run sodium hydroxide solution into the flask until the indicator changes from colourless to a faint permanent pink colour. Add the alkali drop by drop near the end point.

• Repeat as necessary to get two titration readings within 0.20 cm3 of each other.

Actual concentrationsCH3COOH + NaOH CH3COONa + H2O

1. Calculate mass NaOH usedmass = volume x concentration

2. Use balanced equation to identify ratio of chemicals

1 NaOH reacts with 1 CH3COOH3. Calculate formula masses of chemicals

C-12 H-1 O-16 Na-23

4. Find mass of CH3COOH required to neutralise NaOH

40g NaOH reacts with 60g CH3COOH so xg reacts with (x x 60)÷40g

5. Concentration = mass/volume= previous answer/(25÷1000)

CH3COOH + NaOH CH3COONa + H2O• 12.5 cm3 of 4.0 gdm-3 sodium hydroxide neutralises 25.0 cm3

ethanoic acid. Calculate the concentration of ethanoic acid1. Calculate mass NaOH used

mass = volume x concentrationmass = (12.5/1000)x4.0 = 0.050 g

2. Use balanced equation to identify ratio of chemicals1 NaOH reacts with 1 CH3COOH

3. Calculate formula masses of chemicalsC-12 H-1 O-16 Na-23

NaOH = 23+16+1 = 40; CH3COOH = 12+3x1+2x16+1 = 604. Find mass of CH3COOH required to neutralise NaOH

40g NaOH reacts with 60g CH3COOH so 1g NaOH reacts with 60÷40 = 1.5g CH3COOHMass CH3COOH = 0.05x1.5 = 0.075g

5. Concentration = mass/volume= previous answer/(25÷1000)0.075/(25.0/1000) = 3.0 gdm-3