Mole and Chemical Stoichiometry Flashcards
(16 cards)
1 mole is equal to
6.022 x 10^23
Mole is the amount of substance that contains the same number of elementary particles as the number of atoms in exactly ____ of ____
12 grams of Carbon-12
Differentiate empirical and molecular formula
empirical - simplest whole-number ratio
molecular - actual whole-number ratio
How to approach this problem?
An analysis of a compound shows 62.04% C, 10.41% H, and 27.55% O. Determine the formula of the compound, given that its empirical and molecular formulas are identical.
- Convert %composition to grams
e.g. 62.04% C to 62.04 g C - Convert grams to moles
- Get the lowest whole number ratio
A colorless liquid has a composition of 84.1% C and 15.9% H. Find the empirical and molecular formulas of this hydrocarbon. Its molecular weight is 114.2 g/mol.
- Convert %composition to grams
e.g. 84.1% C to 84.1g C - Convert grams to moles
- Get the lowest whole number ratio
- Compute for the empirical formula weight and divide to the molecular weight to get the molecular formula
What are the reagents and products in combustion analysis?
sample (e.g. hydrocarbon) + excess O2 -> CO2 + H2O
How to approach this problem?
Combustion of a 0.200-g vitamin C sample yields 0.2998 g CO2 and 0.0819 g H20. What are the percent composition and empirical formula of vitamin C?
%Composition:
1. g of CO2 and g of H2O to moles
2. moles of CO2 to moles of C and moles of H2O to moles of H
3. moles of C and H to g of C and g of H
4. divide grams of sample then x 100 for %composition
5. %O = 100 - %C - %H
empirical formula
1. Use the moles of C, H, O
2. Get the lowest whole number ratio
Limiting vs excess reactant
Limiting - consumed completely
Excess - remaining reagent
TIP: to get the lowest mol ratio → initial amount / coefficient
How to approach this problem?
Ammonia is produced using the given reaction. In a particular experiment, 0.25 mol NH3 is produced when 0.50 mol N2 is reacted with 0.50 mol H2. What is the %yield?
- Balanced Equation
- Identify the LR
- convert the reagents to moles
- least mole ratio (use ratio & proportion) - Convert LR to product of interest
- %yield = experimental/theoretical x 100
How to approach this problem?
A 1.00-g sample of which compound will produce the greatest amount of CO2 after complete combustion with excess oxygen? CH4, C3H6, C6H14, C8H18
- Balanced equation
- Convert g of sample to mol of CO2
How to approach this problem?
Arrange the following compounds in order of increasing percentage of Cr by mass: CrO, Cr2O3, CrO2, CrO3. The molar masses of Cr and O are 52.00 g/mol and 16.00 g/mol, respectively.
[n x (molar mass of Cr)] / (molar mass of compound)
How to approach this problem?
Which of the following hydrated salts contains the greatest percentage of water by mass?
A. BaCl2 * 2H20
B. Ni(ClO4)2 * 6H20
C. CaCl2 * 6H20
D. CuS04 * 5H20
[n x (molar mass of H2O)] / (molar mass of hydrate)
IMPORTANT: include the H2Os in the molar mass of hydrate
How to approach this problem?
An element X forms two oxides with the formulas are XO3 and X2O3. One of these oxides contains 52% of X by mass and has a molar mass of 99.98. What is the formula of this oxide?
- Let x be the MM of element X
- Make an equation
XO3: x/(x + 16 * 3) = 0.52
X2O3: 2x/(2x + 16 * 3) = 0.52 - Compute for X
- Look at the periodic table for closest MM of element X
How to approach this problem?
A 1.00-g of hydrated potassium carbonate, K2CO3*nH2O, is heated to 250°C to give 0.836 g anhydrous K2CO3. What is the value of n?
- Compute for mass of water
mass of H2O = 1.00 g - 0.836 g - Convert g H2O to mol H2O
- Convert g anhydrous K2CO3 to mol anhydrous K2CO3
- n = mol H2O/mol anhydrous K2CO3
How to approach this problem?
Calcium carbonate decomposes upon heating to calcium oxide and carbon dioxide. What mass of calcium carbonate is required to produce 2.40 L of carbon dioxide measured at STP?
- Balanced equation
- Convert L of CO2 to moles CO2 using ideal gas law and STP condition
STP: 1 bar, 273.15 K - Convert mol CO2 -> mol CaCO3 -> g CaCO3
How to approach this problem?
Assume 0.10 L of N2 and 0.18 L of H2, both at 50 atm and 450°C, are reacted to form NH3. Assuming the reaction goes to completion, identify the reagent that is in excess and determine the volume that remains at the same temperature and pressure.
- Balanced equation
- Identify the LR and ER
- lowest mol ratio = LR - ER used = Convert LR to ER
- ER remaining = initial ER - used ER
IMPORTANT: L can be used as moles since same P and T and the choices are in L too. If not, convert it to moles first using ideal gas law
