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
Importance of % Yield
efficient conversion of reactants to products
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
Steps to make up a standard solution
Stage 1: transfers known mass of solid
- weigh solid sample and container on a 2 d.p
- transfer solid sample to beaker and reweigh sample bottle
- record difference in mass
Stage 2: dissolves in water
- add distilled water
- stir with glass rod/swirl
- until all solid has dissolved
Stage 3: transfer washing and agitation
- transfer to volumetric flask using a funnel
- with washings
- make up to 250cm^3
- shake/invert
Why are any air bubbles removed from the burette tap?
otherwise, the volume of the air bubble woll be counted in the reading resulting in a greater titre
Why is the funnel used to fill the burette removed before starting the titration?
solution drops could fall into the burettte and affect the burette reading which decreases the titre
During the titration, why do you swirl the conical flask and rinse the walls of the conical flask with distilled water?
It ensures all reagents mix and no reagent is left unreacted on the sides of the flask. (This doesn’t affect the number of moles in the conical flask so has no effect on titration)
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.
Empirical Formula Definition
simplest whole number ratio of atoms of each element
Molecular Formula Definition
actual number of atoms of each element
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.
