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Volumetric Calculations. How to analyze titration data and perform dilution calculations. Volumetric procedures. Involve the careful measurement of volumes of solutions

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volumetric calculations

Volumetric Calculations

How to analyze titration data

and

perform dilution calculations

volumetric procedures
Volumetric procedures
  • Involve the careful measurement of volumes of solutions
  • Dissolve materials in water and run reaction in solution - titration in Erlenmeyer flask (designed for swirling to mix reactants)
  • Buret is the measuring device - can read volumes to 0.01 mL
  • Pipets may also be used, especially to transfer solutions - good to 0.01 mL
some terminology
Some terminology
  • Analyte
  • Titrant
  • Standard solution
  • Equivalence point
  • End point

Substance being analyzed

Solution in the buret

Solution of known

concentration

Stoichiometric point

at the equivalence point
At the equivalence point
  • For a 1-to-1 reaction such as:

HCl + NaOH  NaCl + H2O

mole acid = mole base

  • moles = molarity x volume in liters
  • So

Macid x Vacid = Mbase x Vbase

what is the end point
What is the end point?
  • The end point is the point where an indicator changes color.
  • This is the visual sign to stop or end the titration. If the indicator is selected correctly, the change is at or very near the equivalence point!
  • Example: phenolphthalein

colorless in acid

pinkin base

example of concentration calculation
Example of Concentration Calculation
  • A 25.07 mL sample of vinegar is titrated with 37.31 mL of 0.5119 M NaOH. What is the molarity of the acetic acid in vinegar?

Moles NaOH = 0.5119 moles/L x 37.31 mL x 1L/1000mL

Moles acetic acid = moles NaOH = 0.01910 moles

Maa = moles/volume = 0.01910 moles/0.02507 L = 0.7618 M

calculation of analyte mass
Calculation of Analyte Mass
  • A 11.50 g sample of lemon juice is titrated with 0.202M NaOH to determine the mass of the citric acid, a triprotic acid. If it took 36.48 mL of base, what is the mass of C6H8O7? What is the % citric acid by mass?
  • C6H8O7 + 3 NaOH Na3C6H5O7 + 3 H2O
  • Mb x Vb = molb

0.202 M x 0.03648 L = 0.00737 mol

analyte mass continued
Analyte Mass Continued
  • molb/3 = mola 0.00737/3 = 0.00246 mol
  • mola x MMa = ga 0.00246 mol x 192.1 g/mol = 0.473 g C6H8O7
  • % = (mass C6H8O7/mass sample) x 100
  • % = (0.473/11.50) x 100 = 4.11 % C6H8O7
back titration
Back Titration
  • An indirect analysis for antacids, since they are insoluble
  • Antacids, which are bases, are reacted with excess acid and this excess acid is titrated with standard base.

Antacid + HCl (initial)  neutralized antacid + HCl (leftover)

HCl (leftover) + NaOH  NaCl + H2O

back titration calculation
Back Titration Calculation-
  • moles HCl initially = MHCl x VHCl
  • moles HCl left-over = moles NaOH

= MNaOH x VNaOH

  • mole HCl reacted =

moles HCl initially - moles HCl left-over

  • moles antacids - depends on reaction stoichiometry:

CaCO3 + 2HCl  CaCl2 + H2O + CO2

dilution
Dilution
  • Process of adding solvent, usually water, to lower the concentration of a solution
  • Used to make working standards for spectrophotometry
  • Used to bring a sample into range for analysis
  • Typically done using a pipet (TD) and volumetric flask (TC)

Solute is constant. Solvent is being added.

dilution continued
Dilution continued
  • Since the solute is constant, then

molesinitial = molesfinal

  • So the dilution equation becomes

Cinit Vinit = Cfinal Vfinal

where Vfinal = Vinit + Vwater added

dilution calculation
Dilution Calculation

Cinit

Vfinal

  • What initial volume of 0.50 M CuSO4 must be used to make 100 mL of 0.025 M CuSO4 solution?
  • Cinit Vinit = Cfinal Vfinal and rearrange
  • Vinit = Cfinal Vfinal / Cinit

=0.025 M x 100 mL/ 0.50 M = 5.0 mL

Cfinal

dilution continued14
Dilution continued
  • A graduated cylinder is a volume measuring device to contain (TC). Could you do a dilution in it? Explain why or why not.
  • Dilution factor = Vinit/Vfinal

1 mL into 100 mL is a df of 1/100

dilution continued15
Dilution continued
  • Serial dilution – successive dilutions for large scale dilutions performed in steps

A is diluted by 1/100 to B

B is diluted by 1/100 to C

C is 1/10,000 the concentration of A