# Concentration equations

## Presentation on theme: "Concentration equations"— Presentation transcript:

Concentration equations
concentration (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 dm3 = 125cm3 mass (g) = concentration (g/dm3) x volume (dm3) volume (dm3) = mass (g)/concentration (g/dm3)

Calculate the concentration (in g/dm3) of:
3g Na2CO3 in 250cm3 0.5g MgCl2 in 25cm3 Calculate the mass of solute in: 100cm3 2.5g/dm3 CuSO4 (aq) 25cm3 0.1g/dm3 CH3COOH (aq) Calculate the volume of solution needed to provide the mass of solute: 4g KBr from a 16g/dm3 solution = mass/volume (dm3) =3/(250÷1000) = 12g/dm3 = 0.5/(25÷1000) = 20g/dm =concentration x volume = (100÷1000) x 2.5 = 0.25g = (25÷1000) x 0.1 = g = mass/concentration = 4/16 = 0.25dm3 = 250cm3

Making up a standard solution
I am going to make 250 cm3 of 5 g/dm3 sodium carbonate Mass Na2CO3 = concentration (g/dm3) x volume (in dm3) Mass Na2CO3 = 5 x (250/1000) = g Dissolve in about 100 cm3 distilled water Transfer to cm3 volumetric flask Add rinsings to flask – ensures all of the Na2CO3 weighed is in the flask Make up to mark 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 cm3 of each other.

Actual concentrations
CH3COOH + NaOH  CH3COONa + H2O Calculate mass NaOH used mass = volume x concentration Use balanced equation to identify ratio of chemicals 1 NaOH reacts with 1 CH3COOH Calculate formula masses of chemicals C-12 H-1 O-16 Na-23 Find mass of CH3COOH required to neutralise NaOH 40g NaOH reacts with 60g CH3COOH so xg reacts with (x x 60)÷40g 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 acid Calculate mass NaOH used mass = volume x concentration mass = (12.5/1000)x4.0 = g Use balanced equation to identify ratio of chemicals 1 NaOH reacts with 1 CH3COOH Calculate formula masses of chemicals C-12 H-1 O-16 Na-23 NaOH = = 40; CH3COOH = 12+3x1+2x16+1 = 60 Find mass of CH3COOH required to neutralise NaOH 40g NaOH reacts with 60g CH3COOH so 1g NaOH reacts with 60÷40 = 1.5g CH3COOH Mass CH3COOH = 0.05x1.5 = 0.075g Concentration = mass/volume = previous answer/(25÷1000) 0.075/(25.0/1000) = 3.0 gdm-3