Lecture #3.

Answers to quiz questions

• How many micrograms in 1.00 gram? 10^6^ (one million)

• How many nanograms in 1.00 µg? 10^3^ (one thousand)

• How many mL in 1.00 L? 10^3^ (one thousand)

• If 5.00 g of a compound of molar mass 200 g mol^-1^ is dissolved in 100 mL of water, what is the concentration in M (mol L^-1^)? 0.25 M

• What percentage (by weight) of water is hydrogen? 11%

Concentration.

Percent: g per 100 g

ppm: parts per million microgram per gram, mg/kg

parts per billion ppb: microgram per kg, ng per g.

Solution concentrations based on mass per volume. ppm is microgram per mL, ppb is ng per mL.

Concentration in mol/L (M) found by dividing mass of substance in 1 L of solution by the molar mass of the species.

Chemists count atoms and molecules by weighing. So if we want to know how many molecules of ethanol we have, we weigh the ethanol and divide by the weight of one molecule. We calculate the weight of one molecule by adding up the weights of the constituent atoms (2 carbons, 6 hydrogens and 1 oxygen). We find the weights by looking in the period table where we find that the weights we need are 12, 1.0, 16 (to 2 digits). So the weight of a molecule of ethanol is 46. So if we have 23 g of ethanol then we have 23/46 molecules = 0.5. Wait a minute! We were expecting a very big number as molecules are very small. So just what are the units of the weights in the periodic table? Good question: the units are not the weight of just one atom but of a very large number of atoms (in fact the number of atoms in 12 g of the isotope of carbon with 12 neutrons and protons, which is 6.02 x 10^23^, a number known as Avogadro's number). This number also has another special name: a mole (symbol mol). So by dividing the weight by the molecular weight we get the number of molecules in moles.

An important relationship to remember is that the number of moles = (mass in g)/(molar mass).

The molar mass is calculated from the atomic weights found in the periodic table (or elsewhere), which have the units g per mol.

The formula of nitric acid is HNO_3_ (sorry, don't know how to create subscripts (or superscripts) in Dreamweaver.

Scales of working: major components 10 - 100%, minor components 100 ppm - 10%, trace components less than 100 ppm.