Stoichiometry Flashcards
What is the empirical formula of a pure compound (eg. water)?
The simplest whole number ratio between the different elements making up the compound
What is the molecular formula of a pure compound (eg. hydrogen peroxide)?
It states the exact number of different atoms that make up the molecule
Eg. H2O2 (not HO, as with the empirical formula)
What is Avogadro’s number?
6.02 x 10^23
It is the number of atoms in 12 grams of the most abundant isotope of carbon
A mole of atoms or molecules (or any particle) contains an Avogadro number of these particles.
How do you determine how many moles of a compound/element are in a sample?
What is the difference between atomic weight and molecular weight?
(Grams of sample)/(total AMU weight)
Atomic weight is referenced to Carbon-12 for each element, molecular weight is the sum of atomic weights for the atoms in a compound.
How do you determine the composition of a compound by percent mass if you are given the masses of the individual components? (ie determining empirical formula)
(eg. given 40.9 g carbon and 54.5g oxygen and 4.5g hydrogen in 100g of vitamin C to deduce C3H4O3)
- (Mass of components in grams)/(atomic mass) = moles
- divide all numbers of moles for each component by the smallest number of moles (eg. 3.41/3.41=1 and 4.53/3.41=1.33)
- Multiply results to get a whole number for each (eg. 1.33x3=3)
- The whole number received for each component indicates the ratio number in the empirical formulae
How can you determine the percent mass of a compound from combustion analysis?
From combustion analysis you get the mass of CO2 and H20 that is yielded from combustion.
There are 12 g of carbon in 44 g of CO2
(g of CO2)(12/44) = g of Carbon in ___(g) of sample
The same procedure is carried out for hydrogen with the H2O (2.02 g H in 18 g of water)
The results are then multiplied to make a whole number, which will represent the ratios in the empirical formula
How can you determine how much product will be produced from a chemical reaction?
Find the moles of the limiting reactant and then multiply them (if necessary) by the stoichiometric coefficient of the product in question.
eg. 2 moles of LR: 6 moles of product
= 1 mole : 3 moles
eg. (moles of LR)(3) = moles of product
How should redox reactions be balanced? How?
By oxidation state, where electrons are transferred from one atom (or a group of atoms) to another
- Oxidation number is zero in elementary substances (eg. O in O2 or Na in NaCl).
- Oxidation number of monoatomic ions matches their charge (eg. Cl- or Fe3+). This is the only time when ionic charge and oxidation number coincide.
- In a neutral molecule, the sum of the oxidation numbers of all the elements is zero. In a polyatomic ion (eg. SO4 2-), the sum of the oxidation numbers that make up the ion in equal to the charge of the polyatomic ion. The following oxidation numbers then apply (and should be memorized):
H = +1
O = -2 in most compounds, -1 in peroxides (eg. with H2O2), +2 in OF2 and -.5 in superoxides (eg. KO2)
+1 for alkali (first column) metals
+2 for alkaline earth metals (second column)
Al: +3
What is oxidation state and how is it different from ionic charge?
Oxidation state is an artificial concept that is used to perform ‘electron bookkeeping.’
Ionic charge refers to the electron configuration in the valence shell (anions = extra electron, cations = electron deficient)
What are the oxidation numbers of the following atoms?
H =
O =
alkali (first column) metals
alkaline earth metals (second column)
Al:
H = +1
O = -2 in most compounds, -1 in peroxides (eg. with H2O2), +2 in OF2 and -.5 in superoxides (eg. KO2)
+1 for alkali (first column) metals
+2 for alkaline earth metals (second column)
Al: +3
Zn(s) + CuSO4(aq) = ZnSO4(aq) + Cu(s)
Is this a redox reaction? If so, what is being oxidized/reduced and how do we know?
Yes
Copper is reduced (and therefore the oxidizing agent). We know this because it’s oxidation number decreases from +2 (in CuSO4) to 0 (as Cu).
Zn is oxidized from 0 to 2 (reducing agent)
How many moles of sodium ions are there in a 15 mL solution of .3 M Na2SO4 (aq)?
(15 mL is converted to .015)
(.015 L)(.3 mol Na2SO4)(2 mol Na+) divided by (1 L Na2SO4)(1 mol Na2SO4) = .009 moles
AKA: (L)(mol/L) = moles
Give the balanced equation for the combustion of elemental sulfur to sulfur dioxide
S + O2 → SO2
The combustion of elemental sulfur involves a reaction between oxygen (O2) and sulfur (S)
How does elemental sulfur usually exist as a molecule? How is it usually written in an equation?
It exists as S8 molecules (8 sulfurs bonded together). Its reactions are normally written in terms of its empirical formula (S).
Two identical evacuated flasks are filled with different gases to the same pressure and temperature. The first is filled with hydrogen and the second with propane. Compared with the first flask, the flask filled with propane weighs:
Rearranging the ideal gas equation, we get:
n = PV/RT.
Since P, V, and T are the same for both gas samples, n is also the same. However, propane has a molecular weight of 44, while H2 has a molecular weight of 2. Therefore, a molecule of propane weighs 22 times more than a molecule of hydrogen. Consequently, the sample of propane weighs 22 times more than the sample of hydrogen.
