Molarity, ph, and Buffers

Transcription

1 Molarity, ph, and Buffers BTEC 1015 A bit of chemistry review ELEMENT - a substance that cannot be broken down to other substances by chemical reactions ATOM - the smallest unit of matter that still retains the properties of an element COMPOUND - a substance consisting of two or more elements in a fixed ratio 1

2 Sodium Chlorine Sodium chloride ATOMS FORM COMPOUNDS and MOLECULES 2

3 ATOMIC STRUCTURE Electron shell (energy level) consists of moving electrons in orbitals Atomic nucleus composed of protons and neutrons Atomic Mass Atoms of the various elements differ in number of subatomic particles Atoms of the various elements therefore also differ in atomic mass Atomic mass of Na = amu Atomic mass of Cl = amu 3

4 What is a mole? We have no hope of measuring out 500 amu in an ordinary lab. Or even 5,000,000 amu. It s just too tiny an amount. So, we use another, much larger unit, the mole. One mole of water = 6.02 x H 2 O molecules One mole of NaCl = 6.02 x NaCl molecules One mole of NaOH = 6.02 x NaOH molecules One mole of Na = 6.02 x Na atoms What is a mole? One mole of any atom has a mass in grams equal to the atomic weight of the element. mass of one atom of carbon = 12.0 amu mass of one mole of carbon = 12.0 g mass of one atom of calcium = 40.1 amu mass of one mole of calcium = 40.1 g mole is abbreviated mol 4

5 Molecular Weight Molecular weight (m.w.) = weight in amu of one molecule of a substance weight in grams of one mole of a substance What is the molecular weight of NaCl? Atomic weight of Na = amu Atomic weight of Cl = amu amu amu = amu Molecular weight of NaCl is g / mol Label on chemical bottle will generally provide the m.w. (without units) Molarity Molarity is a measure of concentration 1 molar = 1 mole per liter = 1 mol/l = 1 M mole is abbreviated mol molar is abbreviated M 5

6 Molecular Weight vs. Formula Weight Formula weight (f.w.) may be equal to or higher than the m.w., depending on how the chemical is prepared The same amount in grams of two forms may not contain equal numbers of the molecule you re interested in Formula weights: Calcium chloride (anhydrous) = g/mol Calcium chloride dihydrate = g/mol Calcium chloride hexahydrate = g/mol Calculations involving moles You have a 1 M stock solution of CaCl 2. How would you make 100 ml of 0.25 M CaCl 2? 6

7 Preparing a Molar Solution Determine the volume that you want to prepare and express the desired volume in liters (L). Determine the desired concentration, in moles per liter (mol/l = M). Determine the formula weight (f.w.) of the solute (g/mol) Multiply: L * (mol/l) * (g/mol) = g Mix the calculated number of grams with solvent, bringing the solution to final desired volume. In-class practice problems ph: a measure of the [H + ] in a solution ph = -log 10 [H + ] or 1/log 10 [H + ] In pure water, a small number of water molecules ionize (dissociate): HOH H + + OH - [H + ] = M (10-7 M): ph of pure water = -log 10 [10-7 M] = 7 7

8 Acid = proton donor; substance that increases the [H + ] e.g. HCl (hydrochloric acid) in H2O > H + + Cl - Base = proton acceptor; substance that decreases [H + ] e.g. NaOH in H2O > Na + + OH - OH - + H + H 2 O NH 3 (ammonia) + H 2 O NH OH - Buffer = substance that minimizes ph change in solutions Buffers are generally mixtures of weak acids and their conjugate bases. Buffered solutions can absorb the addition of acid or base with only small changes in ph. 8

9 Why are buffers important? In an organism, ph can make the difference between life and death ph of blood is normally ~7.4 ph 6.8 can be lethal Enzymes, which catalyze most biochemical reactions, tend to be highly ph specific In the lab, this means that if your buffer is wrong, your cells may not grow, or reactions won t proceed! Useful ph Ranges of Selected Biological Buffers (25 C, 0.1M) Useful ph Buffer Range MES MOPS HEPES Trizma Tricine TAPS AMPD TABS AMP CAPS

10 How to make a buffer 1. Calculate how much of each component you will add. 2. Dissolve the solutes in a volume of solvent equal to ~ 80% of the desired total volume. 3. Check the ph add a strong acid (e.g. HCl) if you need to lower the ph add a strong base (e.g. NaOH) if you need to raise the ph 4. Once the ph is correct, transfer buffer solution to a graduated cylinder, and bring it up to the desired total volume. Buffer-making tips Use a stir bar when adding acid/base to improve the accuracy of the ph reading. Once you get close to the desired ph, make sure you are adding a dilute form of the strong acid/base, so you don t overshoot the desired ph. If you do overshoot your ph, it s often NOT ok to adjust in the other direction using a strong acid/base; you must start over. Why? 10

11 NaOH + HCl makes NaCl HCl (an acid) in water yields: H 3 O + + Cl NaOH (a base) in water yields: OH - + Na + Combine HCl and NaOH in water: will DONATE an H + will ACCEPT an H + NaOH + HCl NaCl + H 2 O 11