Acids and Bases

Take 2!

pH

• pH depends on the concentration of hydrogen ion (H+)in the solution

–Lots of hydrogen ion = low pH

–Very little hydrogen ion = high pH

What’s concentration?

• Involves the amount of solute (solute is the dissolved

stuff) and the volume of the solution

• Demo of concentration

Vocabulary

• Dilute:

–Verb – to make less concentrated, usually by adding solvent (water)

–Adjective - of or pertaining to a solution that has a low concentration of solute

Diluting acids

• Does the amount of acid change when you add water to it?

• What changes?

• Will the pH change?

• How much?

Molarity (M)

• Concentration is usually measured in units of molarity. It is defined as:

• Molarity =

• Solute could be acid or base moles!

lutionlitersofso

utemolesofsol

Dilution

• When you add water to the acid, you dilute it, but the number of moles of acid DO NOT change. Only the volume of the liquid solution changes.

Molarity Practice

• If 3.00 moles of acid are added to enough water to make 3.00 liters of solution, what is the concentration (molarity) of the acid solution?

Molarity Practice

• If 1.00 moles of acid are added to enough water to make 4.00 liters of solution, what is the concentration (molarity) of the acid solution?

Molarity Practice

• If 4.00 moles of base are added to enough water to make 2.00 liters of solution, what is the concentration (molarity) of the acid solution?

Molarity Practice

• In 0.200 L of a 1.00 M acid solution, how many moles of acid are there?

Molarity Practice

• In 0.500 L of a 0.200 M acid solution, how many moles of acid are there?

Neutralization

• When acid and base are added:

• Acid + base water + a salt

• Remember salts include almost all ionic compounds

Neutralization

• Example

• HCl + NaOH H2O + NaCl

• This one makes water and table salt!

Neutralization

• Example

• HCH3COO + NaOH H2O + NaCH3COO

• This one makes water and a different salt!

Forming water

• Water is made because the H+ from the acid and the hydroxide ion (OH-) from the base get together to form water.

• If you don’t add exactly the same moles of acid and base, you will have more than just water and salt!

Did I add too much Base?

• How could you know whether you’ve exactly cancelled out the acid?

• Ans: TITRATION!!!!!!!

–And – use an indicator or check the pH

Titration

• Add Base carefully to acid and measure EXACTLY how much was used.

• The picture shows a burette tube being used to drop base in acid.

Measuring Volumes

• The meniscus is the curve at the top. Measure from the bottom of the meniscus.

• Notice that the low numbers are on top!

Measuring Volumes

• Notice that you can see if the meniscus is between lines or right on a line.

• Your volume measurement should have 2 decimal places.

Measuring Volumes

• What volume is this indicating?

Phenolphthalein

• It turns pink in base – but is clear in acid.

• The range is 8-9

• This needs to be swirled to see if the pink will go away!

ENDPOINT

• The titration is done at the ENDPOINT.

• That’s where the solution stays pink! (hopefully light pink, otherwise there is too much base!)

Calculations

• The whole point is to calculate the concentration of the acid or base.

• At the endpoint

moles of acid = moles of base

Calculations

• And remember –Moles = molarity X volume

• so

MaVa = MbVb

Molarity X volume = molarity X volume

Practice calculations

• 10.0 mL of a solution of potassium hydroxide was titrated with a 0.10 M solution of hydrochloric acid. 13.5 mL of the acid was required for neutralization. Calculate the concentration of the potassium hydroxide solution.

Practice Calc.

• Write down everything you know.  

• ACID BASE• Hydrochloric Acid               Potassium hydroxide

• Volume used = 13.5mL      Volume used = 10.0 mL

Va Vb • Conc. = 0.10 M                   Concentration =?

Ma Mb • Then set up MaVa = MbVb

Answer

• 0.1M x 13.5 = Mb x 10.0

• Mb = 0.135 M