pH Scale & MolaritypH Scale & Molarity

Unit 1:Unit 1:

BiochemistryBiochemistry

You must know!You must know!

How to interpret the pH scale.How to interpret the pH scale. The importance of buffers in The importance of buffers in

biological systems.biological systems.

pH ScalepH Scale

In an aqueous solution some water In an aqueous solution some water and other molecules tend to ionizeand other molecules tend to ionize– Ex: HEx: H22O O H H++ (hydrogen ion) and OH (hydrogen ion) and OH--

(hydroxide ion)(hydroxide ion) The concentration of HThe concentration of H++ is used to is used to

calculate pHcalculate pH The pH scale ranges from 1 - 14 The pH scale ranges from 1 - 14

pH ScalepH Scale

Between each number of the pH scale the Between each number of the pH scale the HH++ concentration changes 10 fold. concentration changes 10 fold.

It is a negative, logarithmic scaleIt is a negative, logarithmic scalepH = -log [HpH = -log [H++]]

Ex: a solution has a concentration of 10Ex: a solution has a concentration of 10-7-7 Molar HMolar H++ therefore the solution has the pH therefore the solution has the pH = ?= ?

Water is neither acidic nor basic, but Water is neither acidic nor basic, but neutralneutral

AcidsAcids

AcidAcid– Solution that has a high concentration of Solution that has a high concentration of

HH++ ions ions Chemical formula usually begins with HChemical formula usually begins with H Has a pH of 6.9 or lessHas a pH of 6.9 or less Examples: HCl, HExamples: HCl, H22SOSO44, HNO, HNO33, HCH, HCH33OO33

BasesBases

BaseBase– Solution that has a low concentration of HSolution that has a low concentration of H++ (high (high

concentration of OHconcentration of OH--)) Chemical formula usually ends with OHChemical formula usually ends with OH Has a pH of 7.1 or higherHas a pH of 7.1 or higher Examples: NaOH, KOH, Mg(OH)Examples: NaOH, KOH, Mg(OH)22

If an acid is added to water, the pH If an acid is added to water, the pH dropsdrops

If a base is added to water, the pH If a base is added to water, the pH risesrises

BuffersBuffers

BuffersBuffers– Substance that keeps the pH relatively stable Substance that keeps the pH relatively stable

when an acid or base is added.when an acid or base is added. Most biological solutions have a natural Most biological solutions have a natural

ability to buffer: blood, milk, saliva, egg ability to buffer: blood, milk, saliva, egg whitewhite

Chemical reactions in living things Chemical reactions in living things (respiration, photosynthesis) depend on a (respiration, photosynthesis) depend on a stable pHstable pH

Buffers add HBuffers add H++ ions when pH rises or ions when pH rises or remove Hremove H++ ions when pH drops ions when pH drops

MolarityMolarity

Used to determine a concentration in Used to determine a concentration in chemical solutionschemical solutions

1M (one molar) NaCl solution means 1M (one molar) NaCl solution means that 6 x 10that 6 x 102323 molecules of NaCl are molecules of NaCl are dissolved in 1 liter Hdissolved in 1 liter H22O.O.

Mass of a substance (indicated on Mass of a substance (indicated on Periodic Table of Elements): Na = __, Periodic Table of Elements): Na = __, Cl = ___ so NaCl = Na + Cl = 58 Cl = ___ so NaCl = Na + Cl = 58 grams/molegrams/mole

To make a 1M NaCl solution 58g NaCl To make a 1M NaCl solution 58g NaCl will be dissolved in 1L waterwill be dissolved in 1L water