Presentation on theme: "Concentration equations"— Presentation transcript:
1 Concentration equations concentration (g/dm3) = mass (g)/volume (dm3)NOTE: 1dm3 = 1000cm3Conversionscm3 to dm3: vol/1000 e.g. 25.0cm3 = 0.025dm3dm3 to cm3: volx1000 e.g dm3 = 125cm3mass (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 250cm30.5g MgCl2 in 25cm3Calculate 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
3 Making up a standard solution I am going to make 250 cm3 of 5 g/dm3 sodium carbonateMass Na2CO3 = concentration (g/dm3) x volume (in dm3)Mass Na2CO3 = 5 x (250/1000) = gDissolve in about 100 cm3 distilled waterTransfer to cm3 volumetric flaskAdd rinsings to flask – ensures all of the Na2CO3 weighed is in the flaskMake up to markReading 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 cm3 of each other.
5 Actual concentrations CH3COOH + NaOH CH3COONa + H2OCalculate mass NaOH usedmass = volume x concentrationUse balanced equation to identify ratio of chemicals1 NaOH reacts with 1 CH3COOHCalculate formula masses of chemicalsC-12 H-1 O-16 Na-23Find mass of CH3COOH required to neutralise NaOH40g NaOH reacts with 60g CH3COOHso xg reacts with (x x 60)÷40gConcentration = 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 acidCalculate mass NaOH usedmass = volume x concentrationmass = (12.5/1000)x4.0 = gUse balanced equation to identify ratio of chemicals1 NaOH reacts with 1 CH3COOHCalculate formula masses of chemicalsC-12 H-1 O-16 Na-23 NaOH = = 40; CH3COOH = 12+3x1+2x16+1 = 60Find mass of CH3COOH required to neutralise NaOH40g NaOH reacts with 60g CH3COOHso 1g NaOH reacts with 60÷40 = 1.5g CH3COOH Mass CH3COOH = 0.05x1.5 = 0.075gConcentration = mass/volume= previous answer/(25÷1000) 0.075/(25.0/1000) = 3.0 gdm-3