Chapter 4 Flashcards
In the submicroscopic scale, what the stoichiometric coefficients in the equation refer to ?
atoms, molecules, or formula units
In the macroscopic scale, what the stoichiometric coefficients in the equation refer to ?
the number of moles of each reactant and product.
In stoichiometry, the coefficient in a balanced chemical equation thus allow us to relate __________________________ in a chemical reaction to the ________________________ in that chemical reaction
the amount (moles) of one substance involved
amount (moles) of another substance involved
How do we do stoichiometry calculations in a mole to mole problem?
example: Al + HCl → AlCl3 + H2
How many moles of HCl are needed to react with 0.87 moles of Al?
Step 1: Be sure that the equation is balanced. If not balance it.
not balanced: Al + HCl → AlCl3 + H2
Balanced: 2Al + 6HCl → 2AlCl3 + 3H2
Step 2: find the mole ratio.
2Al:6HCL=2:6
2Al:2AlCl2=2:2
2Al:3H2=2:3
Step 3: Find the moles of the given
0.87 moles of Al will react with HCl
Step 4: Calculate the moles using the ratios
0.87 mol Al X 3 mol HCl/ 1 mol Al
= 2.6 mol HCl
How do we do stoichiometry calculations in a mass to mass problem?
example: Al2O3 → Al + O2
How many grams of Al can be created decomposing 9.8g of Al2O3?
Step 1: Be sure that the equation is balanced. If not balance it.
not balanced: Al2O3 → Al + O2
Balanced: 2Al2O3 → 4Al + 3O2
Step 2: find the mole ratio.
2Al2O3 : 4Al= 2:4
2Al2O3 : 3O2= 2:3
Step 3: Calculate the moles of the given (mol/g)
9.8g Al2O3 x (1mol Al2O3/102g Al2O3) = 0.096 mol Al2O3
Step 4: Calculate the moles using the ratios
0.096 mol Al2O3 x (4 mol Al/2 mol Al2O3) = 0.19 mol Al
Step 5: Calculate the mass using the new moles
0.19 mol Al x (27g Al/1 mol Al) = 5.1g Al
How do we do stoichiometry calculations in a mass-volume problem?
K + H2O→ KOH + H2
How many liters of H2 are created from the reaction of 20.0g K?
knowing that H2 is 22.4L
Step 1: Be sure that the equation is balanced. If not balance it.
Not balanced: K + H2O→ KOH + H2
Balanced: 2K + 2H2O→ 2KOH + H2
Step 2: find the mole ratio.
2K : 2H2O = 2:2
2K : 2KOH = 2:2
2K : 1H2 = 2:1
Step 2: Find the Mass of the Given
20.0g K are used in the reaction
Step 3: Calculate the moles of the given (mol/g)
20.0g K x (1 mol K / 39g K) = 0.513 mol K
Step 4: Calculate the moles using the ratios
0.51 mol K x (1mol H2 /2mol K) = 0.266mol H2
Step 5: Calculate the volume using the new moles
0.266 mol H2 x (22.4L H2 /1mol H2) = 5.75L H2
How do we do stoichiometry calculations in a volume-volume problem?
S2 + O2 → SO2
How many liters of SO2 will be produced from 26.9L O2?
knowing that 1 mol of O2= 22.4L
Step 1: Be sure that the equation is balanced. If not balance it.
Not balanced: S2 + O2 → SO2
Balanced: S2 + 2O2 → 2SO2
Step 2: find the mole ratio.
2O2 : 1S2 = 2:1
2O2 : 2SO2 = 1:1
Step 3: Find the volume of the given
26.9L O2
Step 3: Calculate the moles of the given
26.9L O2 x (1 mol O2 / 22.4L) = 1.20 mol O2
Step 4: Calculate the moles using the ratios
1.20 mol O2 x (1mol SO2 /1mol O2) = 1.20 mol SO2
Step 5: Calculate the volume using the new moles
1.20 mol O2 x (1mol SO2 /1mol O2) x (22.4L /1mol) = 26.9L SO2
What is a limiting reactant?
its amount determines, or limits, the amount of product formed
What are the steps to find the limiting reactant?
example: 2 HCl (aq) + Ca(OH)2 (s) →CaCl2 (aq) + 2 H2O (l)
How many grams of water are made when 0.656 g each of hydrochloric acid (HCl) and calcium hydroxide are mixed?
in the answer:
- Steps 1 and 2 are to find limiting reagent
- Steps 3 to 5 are to find the answer to the previous question
Step 1: Convert the mass of each reactant into moles of product
- 656g HCl X (1 mol HCl / 36.46g of HCL) X ( 2 mol H2O / 2 mol HCl) = 0.0180mol H2O
- 656g Ca(OH)2 X (1 mol Ca(OH)2 / 74.10 g Ca(OH)2) X (2 mol H2O / 1 mol Ca(OH)2) = 0.0177mol H2O
Step 2: Find which reactant produced the less product
- 0180mol H2O are produced with HCl
- 0177mol H2O are produced with Ca(OH)2
So Ca(OH)2 is the limiting reagent HCl is in excess
Step 3: Calculate the mass of product produced
0.0177 mol H2O X ( 18.02 g H2O / 1 mol H2O) = 0.319 g H2O
Step 4: Calculate the moles of excess reactant used and moles that remain.
0.656g HCl X (1 mol HCl / 36.46 g)= 0.0180 mol HCl at start
- 0177 mol H2O X (2 mol HCl / 2 mol H2O)= 0.0177 mol HCl used
- 0180 mol HCl at start – 0.0177 mol HCl used = 0.0003 mol HCl remain
Step 5: Calculate the mass of excess reactant that remain.
0.0003 mol HCl X (36.46 g HCl / 1 mol)= 0.01 g HCl
What is the formula to find the percent yield?
% yield = (actual/theoretical) x 100
How do we determine the empirical formula of a compound by combustion?
The empirical formula of a compound can be determined if the percent composition of the compound is known.
One chemical methods that works well for compounds that burn in oxygen is analysis by combustion.
Each element in the compound combines with oxygen to produce the appropriate oxide
How do we measure concentrations of compounds in solution ?
by molarity:
molarity =(amount of solute x (mol))/(volume of solution (L))
Molarity refers to the amount of _________ per ____
solute per liter
How do we calculate the pH?
PH= -log(H+)
PH= -log(molarity of the acid)
A pH of 7 is ____________
neutral
A pH less than 7 is __________
acidic
A pH greater than 7 is_________
basic
What titration is used for
to determine the quantity of oxalic acid present in a given mass of sample
What was the concentration of the HCl?
A 25 ml solution of 0.5 M NaOH is titrated until neutralized into a 50 ml sample of HCl.
Step 1 - Determine [OH-]
Every mole of NaOH will have one mole of OH-. Therefore [OH-] = 0.5 M.
Step 2 - Determine the number of moles of OH-
Molarity = # of moles/volume
# of moles = Molarity x Volume # of moles OH- = (0.5 M)(.00025 L) # of moles OH- = 0.000125 mol
Step 3 - Determine the number of moles of H+
When the base neutralizes the acid, the number of moles of H+ = the number of moles of OH-. Therefore the number of moles of H+ = 0.0000125 moles.
Step 4 - Determine concentration of HCl
Every mole of HCl will produce one mole of H+, therefore number of moles of HCl = number of moles of H+.
Molarity = # of moles/volume
Molarity of HCl = (0.000125 mol)/(0.00050 L)
Molarity of HCl = 0.25 M
Answer
The concentration of the HCl is 0.25 M.
What is a standardization of an acid or a base?
The procedure by which the concentration of an analytical reagent is determined accurately
How do we standardize an acid or a base?
2 approaches
1- to weigh accurately a sample of a pure, solid acid or base and then titrate this sample with a solution of the base or acid to be standardized.
2- standardizing a solution is to titrate it with another solution that is already standardized.
Standardizing and acid by titration
Problem:
Na2CO3, is a base, and an accurately weighed sample can be used to standardize an acid.
A sample of Na2CO3 (0.263g) requires 28.35 mL of aqueous HCl for titration to the equivalence point.
What is the concentration of the HCl?
Step 1: Be sure that the equation is balanced. If not balance it.
Na2CO3 + 2HCl→ 2NaCl + H2O + CO2
Step 2: Calculate the amount of the base, Na2CO3, from its mass and molar mass.
0.263 g Na2CO3 X (1 mol Na2CO3 / 106.0 g Na2CO3)= 0.00248 mol Na2CO3
Step 3: use the stoichiometric factor to calculate the amount of HCl in 28.35 mL
0.00248 mol Na2CO3 X (2 mol HCl required / 1 mol Na2CO3 available) = 0.00496 mol HCl
Step 4: calculate the concentration of HCl solution by dividing the amount of HCl by the volume of HCl used in the titration
HCl= 0.00496 mol HCl / 0.02835 L= 0.175 M HCl
When can we determine the molar mass of an unknown substance by titration?
If the unknown substance is an acid or a base
Determining the molar mass of an acid by titration.
Problem:
To determine the molar mass of an unknown organic acid, HA, a 1.056g sample of HA is titrated with standardized NaOH.
calculate the molar mass of HA assuming the acid react with 33.78mL of 0.256M NaOH according to this equation
HA + NaOH → NaA + H2O
Step 1: calculate the amount of NaOH used in the titration
Concentration(NaOH) X Volume(NaOH)= (0.256 mol / 1L) X 0.03378 L= 0.00865 mol NaOH
Step 2: The amount of NaOH used in the titration is the same as the amount of acid titrated.
0.00865 mol NaOH X (1 mol HA / 1 mol NaOH)= 0.00865 mol HA
Step 3: calculate the molar mass of HA
molar mass of acid = ( 1.056 g HA / 0.00865 mol HA) = 122 g/ mol HA
How do we determine the equivalence point of an oxidation-reduction reaction?
by titration
Using an oxidation-reduction reaction in a titration
Problem:
The iron in a sample of an iron ore can be converted quantitatively to the iron(ii) ion, Fe^(2+), in aqueous solution, and this solution can then be titrated with aqueous potassium permanganate, KMnO4.
The balanced, net ionic equation for the reaction occurring in this titration is
MnO4 + 5 Fe + 8 H3O → Mn + 5 Fe + 12 H2O
A 1.026g sample of iron-containing ore requires 24.35 mL of 0.0195 M KMnO4 to reach the equivalence point. What is the mass percent of iron in the ore?
Step 1: convert milliliters to liters
24.35 mL/1000= 0.02435 L
Step 2: calculate the amount of KMnO4
Concentration(KMnO4) X Volume(KMnO4) =
(0.0195 mol KMnO4 / 1L) X 0.02435= 0.000475 mol KMnO4
Step 3: use the stoichiometric factor to calculate the amount of iron (ii) ion
0.000475 mol KMnO4 X (5 mol Fe / 1 mol KMnO4)
= 0.00237 mol Fe
Step 4: calculate the mass of iron
0.00237 mol Fe X (55.85g Fe / 1 mol Fe) = 0.133g Fe
Step 5: determine the mass percent
- 133g Fe / 1.026 g sample = 0.129
- 129 X 100= 12.9% iron
What spectrophotometry is used for?
to measure, quantitatively, the extent of light absorption and to relate this to the concentration of the dissolved solute
When a solution have a deeper colour than another, which one is more concentrated?
The deeper one. Because the intensity of the color is a measure of the concentration of the material in the solution.
What is transmittance in spectrophotometry?
Transmittance (T) is the ratio of the amount of light transmitted by or passing through the same (P) relative to the amount of light that initially fell on the sample (the incident light, P_o).
How do we measure transmittance in spectrophotometry?
(T) = (P/P_0) = intensity of transmitted light / intensity of incident light
What is absorbance in spectrophotometry?
Absorbance is defined as the negative logarithm of its transmittance.
How do we measure the absorbance in spectrophotometry?
A= -log (P / P_o)
When the concentration increase the absorbance, A, of a sample __________
increases
When the transmittance of a solution decreases the absorbance ______________
increases
What is the Beer-Lambert Law?
This law states that there is a linear relationship between the absorbance and the concentration of a sample.
How do we calculate the Beer-Lambert Law?
Absorbance= molar absorptivity X path length X concentration
A=ε x l x c
A= the absorbance of the sample, is a dimensionless number
ε= a proportionality constant, is called the molar absorptivity. It is a constant for a given substance, provided the temperature and wave-length are constant. It has units of L/mol ∙ cm.
l and c have the units of length (cm) and concentration (mol/L), respectively.
What are the 4 steps in carrying out a spectrophotometric analysis.
Step 1: Record the absorption spectrum of the substance to be analyzed
Step 2: Choose the wavelength for the measurement
- In theory you could choose any wavelength for quantitative estimations of concentration
- The magnitude of the absorbance is important, especially when you are trying to detect very small amounts of material
Step 3: prepare a calibration plot
-This consists of a plot of absorbance versus concentration for a series of standard solutions whose concentrations are accurately known
Step 4: Determine the concentration of the species of interests in other solutions
