6. Amount of Substance Flashcards
What is Avagadro’s Constant?
number of atoms in 12g of 12-C
Relative Atomic Mass (Ar) Equation
Relative Molecular Mass (Mr) Equation
Equation to work out the number of particles
Moles x Avagadro’s Constant
Equation to work out mass
Mr x Moles
Equation to work out concentration (mol dm-3)
moles / volume
% Yield Equation
(Actual Yield x 100) / Theoretical Yield
% Atom Economy
1 of the only instances when you use the big number when calculating
% error (% uncertainty)
Ideal Gas Equation
PV=nRT
P = pressure in Pa, n = number of moles, V =. volume measured in m^3, R = molar gas constant (always 8.31 K K^-1 mol^-1 = always given), T = tempurate in K
How do you get from m^3 to cm^3?
How do you get from cm^3 to m^3?
1) x 10^6
2) ➗ 10^6
How do you get from m^3 to dm^3?
How do you get from dm^3 to m^3?
1) x 10^3
2) ➗10^3
How do you get from dm^3 to cm^3?
How do you get from cm^3 to dm^3?
1) x 10^3
2) ➗ 10^3
How do you get from ℃ to K?
How do you get from K to ℃?
1) +273
2) -273
Importance of atom economy
increases the mass of atoms in the reactants that are turned into products
Example: Atom Economy
Desired product = NaClO. Calculate atom economy.
2NaOH + Cl2 → NaCl + NaClO + H2O
Example: % Yield
13.5 g of Al reacts with excess O2 to form 18.4g of Al2O3. Calculate % yield.
2Al + 1.5 O2 → Al2O3
Example 2: % Yield (QP pg12, Q15)
After reacting some Zn metal with some excess H2SO4, a student collected 40.8g of ZnSO4.7H2O crystals. What was the original mass of Zn used?
Example: Reaction in Solution
25cm^3 of NaOH solution with a conc of 0.22 mol dm^-3 is reacted against HNO3 of unknow conc. The vol of HNO3 used was 23.35cm^3. Calculate the concentrattion of HNO3.
NaOH + HNO3 → NaNO3 + H2O
Example: Titration Calculations (QP - pg21, Q1)
A student dissolved an uknown mass of NaOH in water to make 200cm^3 of an aq solution. A 25cm^3 sample of this NaOH solution is placed in a conical flask and is titrated with 0.15 mol dm^-3 H2SO4.
2NaOH (aq) + H2SO4 (aq) → Na2SO4 (aq) + 2H2O (l)
Calculate the original mass of NaOH required to make the original solution
Example: % error
A burette was used to measure a titre of 30 cm^3. The error of each reading was ±0.1 cm^3. What’s the % error?
What is a way to decrease %E in a titre measurement?
(increase the measurement made from the burette)
- increases the conc of the solution in the conical flask
- decrease the conc of the solution in the burette
Justification: as a larger titre volume will be needed
Boron Trichloride can be prepared as shown by the following equation:
B2O3 (s) + 3C (s) + 3Cl2 (g) → 2BCl3 (g) + 3CO(g)
A sample of B2O3 was reacted completely with C and Cl. The 2 gases produced occupied a total volume of 5000cm^3 at a pressure of 100kPa and a temperature of 298K.
Calculate the mass of B2O3 that reacted. Give your answer to 3 s.f. Gas Constant = 8.31 J K^-1 mol^-1.
Example: Empirical Formula
10.01g of a white solid contains 4.01 g of Calcium, 1.20g of Carbon and 4.80g of Oxygen. Calculate the empirical formula.
Use MADR: M=mass, A=Ar, D=divide by smallest, R =ratio
Example 2: Empirical and Molecular Formula
An unknown compund consists of 55% C, 9% H and the remainder is O. Mr=88. Calculate the empirical and molecular formula.
Barium Chloride has the formula BaCl2. It’s often found as Barium Chloride Hydrate with a formula BaCl2.xH2O, where x is an integer. 1 sample of BaCl2.xH2O is composed of 82.25% BaCl2 and 14.75% H2O. Determine the value of x in BaCl2.xH2O.
