PH Scale, PH Formula, Titration Formula, Error Propogation, and Uncertainty Formula

pH scale

• The pH scale expresses the strength of an acid or base.

  • It measures [ H₃O⁺ ] or [H⁺] (the hydrogen ion concentration.)

• The pH scale ranges from 0 – 14.

  • pH < 7 is acidic

  • pH > 7 is basic

  • pH = 7 is neutral

    • A pH of 7.3 is slightly basic.

    • Only exactly 7 is neutral.

The pOH Scale

• The pOH is the REVERSE of the pH scale!

• pOH tells you how basic a solution is

• Scale from 14 to 0

  • The lower the pOH, the more basic  the solution.

  • The higher the pOH, the more acidic the solution.

• Never use pOH to figure out if something is acidic or basic!  ALWAYS convert to pH!

pH and pOH Formulas

pOH + pH = 14

How do we calculate how acidic or basic a solution is?

• pH measures the concentration of hydrogen ions [H⁺]

• pOH measures the concentration of hydroxide ions [OH^-]

Calculating [H⁺] or [OH^-]

• To solve for the concentration of H⁺ or OH^-, we use antilog!

Titration

• An experimental technique used to determine the concentration of an unknown solution.

• This is done by using a known concentration of one substance and adding it to another solution of an unknown concentration.

Titration Calculations

• We use the same equation for titrations as for dilutions:

  M₁V₁ = M₂V₂

• For titrations, we’re solving for the volumes at which the moles of acid equal the moles of base present.

Types of Experimental Errors

• Random

  • Can go in either direction

  • Typically from instrumentation

  • Give us ± value

• Systemic

  • Go in a particular direction

  • Flaw in instrument

    • Fix with calibration

  • Flaw in method

    • Separation lab – mass would be too high if sand or salt is still wet

Uncertainty in Calculations- Addition & Subtraction

• When doing a calculation that involves adding or subtracting, add absolute uncertainties directly from instrument

• Ex:

  1.2 cm ± 0.1 cm

+2.5 cm ± 0.1 cm

  3.7 cm ± 0.2 cm

Uncertainty in Calculations- Multiplication & Division

• When doing a calculation that involves multiplying and/or dividing

  • Find the percent uncertainty of each measurement 

    • (uncertainty ÷ measurement) * 100

  • Add all of the percent uncertainties together

  • Value can be reported as percent.

  • Density = m/V

  mass =   4.35 g ± 0.2% = 0.78 g/ml ± 2%

  Volume =   5.6 mL ± 2%

Uncertainty in Calculations- Averages

• When doing the average of calculated values, standard deviation is used for propagation of error