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Concentration equations

Concentration equations. concentration (g/dm 3 ) = mass (g)/volume (dm 3 ) NOTE: 1dm 3 = 1000cm 3 Conversions cm 3 to dm 3 : vol/1000 e.g. 25.0cm 3 = 0.025dm 3 dm 3 to cm 3 : volx1000 e.g. 0.125dm 3 = 125cm 3 mass (g) = concentration (g/dm 3 ) x volume (dm 3 )

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Concentration equations

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  1. 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. 0.125dm3 = 125cm3 mass (g) = concentration (g/dm3) x volume (dm3) volume (dm3) = mass (g)/concentration (g/dm3)

  2. 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/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

  3. 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) = 1.325 g • Dissolve in about 100 cm3distilled water • Transfer to 250.0 cm3volumetric 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

  4. 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.

  5. 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)

  6. 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 = 0.050 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-23NaOH = 23+16+1 = 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 CH3COOHMass CH3COOH = 0.05x1.5 = 0.075g • Concentration = mass/volume = previous answer/(25÷1000)0.075/(25.0/1000) = 3.0 gdm-3

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