chem 11 moles

what does a chemical formula represent

the number and type of each atom present in a compound or element (recall law of definite proportions)

law of conservation of mass

mass is neither created nor destroyed in a chemical reaction

how can the mass of a formula unit be determined

by adding together the masses of each atom present

what does the term formula mass apply to

atoms molecules and ionic compounds (metal/nonmetal atoms)

what does term molecular mass apply to

only covalent compounds (non metal + non metal)

relative mass

comparing the mass of one object to the mass of another

a certain number of oranges have a mass of 3000 g and an equal number of grapefruits have a mass of 5000 g. What fraction of the mass of the grapefruits do the oranges have

3000g/5000g = 0.6

ratio represents relative mass of an orange compared to a grapefruit. one orange has 0.6 the mass of the grape fruit. note: mass of individual fruit is unknown.

unit to measure mass of atoms

atomic mass unit, amu (SI unit symbol u)

how do they determine the mass of atoms

the carbon-12 atom is assigned a mass of 12u and the masses of all other atoms are relative to this standard.

what is one ‘u’ equal to relative to a single carbon atom

one ‘u‘ is equal to 1/12 the mass of a single carbon-12 atom. 

what is a mole of an element

defined as the number of atoms in exactly 12g of carbon

1 mole + 6.02 × 10²³ atoms

why does carbon have the weight 12.011 and not 12u

the presence of isotopes

what are isotopes

atoms of the same element (same # of protons) but with a difference in # of neutrons.

most elements occur as a mixture of a different what

isotopes

what is the atomic mass of an element based on

atomic mass is a weighted average based on abundance of each isotope

how many 12/6 C atoms have a mass of exactly 12 grams

6.02212 ×10²³ atoms

what is a mole

a way of measuring the amount of things, like a dozen. but where a dozen means 12 things, a mole means 602,000,000,000,000,000,000,000 (6.02×10²³ things)

avogadros number

6.02 ×10²³

how to convert from moles to particles

multiply by avogadros number

Number of moles/1
x
6.02 x 10*23/moles
=
number of particles

convert from particles to moles

divide by avogadros number

particles / 1
times
1 mole/6.02 x 10*23 particles
=
moles

convert from mass to moles

divide by formula weight

mass (divided by) molar mass

= ()g/1 x 1 mol/()g

= ()mols

memory aid for converting number of particles to moles to mass

particles divided by na = moles, multiplied by fw = mass

mass divided by fw = moles, times na = particles

percent composition

mass of an element in a compound compared to mass of the compound, expressed as a percentage.

how to find percent composition?

mass of each compound / total

mass of compound x 100%

OR

formula mass of each element/total formula mass of the compound x 100%

how to find empirical formula?

from experimental data. by determening the number of moles of each element present in a chemical reaction the ration in which the elements combine can be found

what does the empirical formula represent?

smallest whole number ratio of atoms in a compound

eg. HO instead of H202

what does the molecular formula represent

the actual number of atoms in a molecule

eg H202

how to determine the empirical formula for the mass of elements?

1. find mass of each element in compound

2. calculate number of moles of each element in the compound

3. divide the number of moles of each element by the smallest number of moles present to get the ratio of atoms in the compound

how to find the molecular formula from the empirical formula?

molecular formula: AxnBn (always same multiple, n, of the empirical formula. To find multiple, divide molar mass of compound by molar mass of empirical formula)

what do gas volumes vary with

temperature (T) and pressure (P)

what is STP and what are the values?

Standard Temperature and Pressure

Standard T = mp of water at sea level = 273 K (0 degrees C)

Standard P = atmospheric pressure at sea level 101.3 kPa = 1 atm

what did chemists do in the early 19th century?

reacted gases together and compared ratios of gas volumes and masses

what did dalton discover

atoms react in fixed simple whole number ratios

what is the law of combinding volumes from Gay-Lussac?

equal volumes of different gases at the same T and P contain the same number of particles

what is molar volume?

1 mole of any gas (and only gases) at STP has a volume of 22.4L

what is molarity or concentration

the number of moles in a given volume of solution describes the concentration of that solution. the molar concentration or molarity (M) of a solution is the number of moles of dissolved substance (solute) per litre of solution.

how to calculate molarity?

M = N/V

N = number of moles. V = volume of solution in litres. M = molarity (mol/l or M)

whats the shorthand for molar concentration?

a square braket

eg. molarity or molar concentration of NaCl = [NaCl] = Mol/L or M

what is the first step for preparing a solution

calculate the mass of solid required

what is the second step for preparing a solution

weigh out the solid on a balance

what is the third step for preparing a solution

transfer the solid quantitatively (all of it!) to a volumetric flask

what is the fourth step for preparing a solution?

add some water to dissolve (DO NOT FILL UP!) swirl until mostly dissolved

what is the fifth step for preparing a solution

add more water until the bottom of the meniscus touches the marked line (add by drop when approaching the line)

what is the sixth step for preparing a solution?

mix well (invert the flask with a stopper a few times.)

avogadros hypothesis

equal volumes of all gases, at the same temp and pressure, have the same number of molecules.

molar mass

mass of one mole of a substance (measured g/mol)