1.2 Amounts of substances Flashcards
What is the definition of Relative Atomic Mass?
average mass of an atom of an element
on a scale where an atom of carbon-12 has a mass of 12
Relative Molecular Mass
Average mass of a molecule on a scale where an atom of carbon-12 has a mass of 12
Diff between Relative Formula mass and Mr
For ionic compounds (that do not exist as molecules) the term relative formula mass is used instead.
Formula of Complex Ion Manganate (VII)
MnO4-
Complex Ion Chromate
CrO2^-
Complex Ion Dichromate
Cr2O7^2-
Why are ionic compounds in Relative Formula Mass?
Because ionic compounds don’t really form molecules (they form large lattices) we cannot talk about molecular units.
Chromium (III) sulphate formula
Cr2(SO4)3
Ammonium nitrate
NH4NO3
Copper (II) sulphide
CuS3
Diff btw sulphide and phate
Ide- just ion with oxygen
ate- with oxygen SO4
Deduce the formula of the compound that contains 2+ ions and 3− ions that both have the same electron configuration as argon.
Originally had two more electrons than Ar, 2+ ion = Ca2+
Originally had three less than Ar, 3- ion = P3-
Ca3P2
Significant figures for calculations involving
multiplication / division
Your final answer should be given to the same number of significant figures as the least number of significant figures in the data used.
Significant figures for calculations involving
addition/subtraction ONLY
Here the number of significant figures is irrelevant – it is about the place value of the data. We round our answer to the data with the fewest number of decimal places.
answer cannot have more decimal places than
either of the original numbers
SI Units
Amperes, Kelvin (temp), Kilograms (mass), Candela (light), Moles, Seconds (time), Metres
Calculate Mr of aspirin (search up skeletal structure)
C9H804
What are the two formulas for moles that link?
moles = mass (g)/ Mr
moles in solution = concentration c (mol dm-3) x volume V (cm^3)
One mole of any substance contains the same amount of particles…
6.022 x 10^23
What is the limiting factor?
In the real world of chemistry, it is rare that we react the exact right amount of chemicals together. Usually, we have more than we need of one of the reactants and so it doesn’t all react – it is in excess.
Sometimes in calculations, we need to work out if one of the reactants is in excess. The reactant that is not in excess is sometimes called the limiting reagent.
Basically- reactant that is used up the fastest and produces the least amount of product- reactant not in excess
Three step to work out limiting factor and do example from white cards
1) Find masses and Mrs
2) Mol for one reactant
3) Look at big no. to find theoretical moles for other reactacnt- multiply moles by big no.
4) Find actual moles for reactant
5) Compare- if actual is less than theoretical then reactant is limiting
6) Limiting (actual) moles x big no. in front of product is product moles
Steps to work out the empirical formula
Write the % or mass of each element
Divide each % or mass by that element’s Ar
Divide each answer by the smallest answer from above to find the simplest ratio
Def:The lowest whole number ratio of atoms of elements in a compound
Molecular formula= molar/empirical
Then multiply answer by empirical
Divide the actual molar mass by the mass of one mole of the empirical formula.
Caffeine has a molar mass of 194 g. what is its molecular formula? Empirical formula = C4H5N2O1
molecular formula
= molar mass/ empirical formula mass
194/97=2
2 x C4H5N2O1= C8H10N402
What do the gas laws look at relationships between?
Temp, pressure and volume
Boyle’s Law
Boyle’s law -describes the relationship between PRESSURE and VOLUME.
The product of pressure multiplied by the volume of a gas in an enclosed container remains the same if either the pressure or volume changes and the temperature remains constant so…
P↑, V↓ and vice versa
If you double the pressure, vol of gas halves
PV= constant
Charles’ Law
TEMP and VOLUME
If you DOUBLE the absolute temperature of a gas, the volume DOUBLES.
T↑V↑
V/T= constant
Gay-Lussac’s law
Relationship between TEMP and PRESSURE
The pressure of a gas VARIES DIRECTLY with the absolute temperature of the gas when volume remains constant. (e.g think of empty deodorant can- is cold when empty)
T↑P↑
P/T= constant
Combined Gas Laws- equation, what constant depends on, units etc
PV/T= constant
constant depends on the conditions used e.g- temp, press, moles of gas
If we have one mole of gas at a standard temp + press we can call this the gas CONSTANT represented by R
PV= nRT
R (given in Q)= 8.31 JK -1 mol-1
P= Pascals (Nm-1)
V=m^3
T=K
Ideal gas equation
PV= nRT
R (given in Q)= 8.31 JK -1 mol-1 P= Pascals (Nm-1) V=m^3 T=K n= no. of moles
Ideal Gas Def
In an ideal gas…
- Collisions are perfectly elastic
- No intermolecular attractive forces (i.e all internal energy = kinetic energy)
- Therefore does not condense into a liquid when cooled.
In reality, there are no gases that fit this definition perfectly. We assume that gases are ideal to simplify our calculations.
Molar Volume def, how it can be calculated, rtp, equation, what it is at rtp
The volume occupied by 1 mole of a gaseous substance is called its molar volume.
Molar volume can be calculated from the density of the gas and its formula mass, Mr.
Since volume depends on both temperature and pressure, molar volume is generally measured at “room temperature and pressure”, rtp, which is about 20oC and 1 atmosphere.
DMV-
Density = mass/volume
rearranging this we get….
Molar volume = mass of 1 mole (Mr) /density at rtp.
The molar volume of any gas at rtp is approximately 24 dm3. Or, since there are 1000 cm3 in 1 dm3 The molar volume is also 24000 cm3.
What can we calc from a balanced equation
How much of any given product can be made.
In the real world of industry there are 2 further important factors
- The yield of a reaction. (Usually expressed as a %)
- Atom economy. This is a theoretical value.
Why might the theoretical yield fall short? What is the theoreyical yield equation?
Not all product is recovered (e.g. spattering, loss)
Reactant impurities (e.g. weigh out 100 g of chemical which has 20 g of junk)
A side reaction occurs
The reaction does not go to completion
Inaccuracy in measurement
(no. of moles of product/theoretical max no. of moles of product) x100
(actual/theoretical)x 100
What is the % yield of NH3 if 40.5 g NH3 is produced from 20.0 mol H2 and excess N2?
Step 1: write the balanced chemical equation
N2 + 3H2 → 2NH3
Step 2: Calculate theoretical yield (actual is given): 1 mol H2 ≡ 2/3 mol NH3 20.0 mol H2 ≡ 13.3 mol NH3 Mass of NH3 13.3 x 17.0=227
Step 3: Calculate % yield
act/theo
(40.5/227)x100=17.87%
cm3 to dm3 to m3
x1000→x1000→
/1000←/1000←
Two moles calcs (one spec for titrations)
mol/mass = conc x vol mol= mass/Mr
What does M mean in terms of conc?
Concentration is usually measured in mol/dm3 (moldm-3)
Sometimes concentration is referred to as ‘molar’
i.e 2M NaOH
This means the same as 2 moldm-3 of NaOH
You have an unknown acid. How can you determine the concentration of this unknown acid?
We can use an alkali to react with the acid.
When the acid has been neutralised, the amount of alkali added allows us to determine the concentration of acid.
You have an unknown concentration of HCl acid. 25.0cm3 of this acid is neutralised by 30.0cm3 of 0.10M NaOH. What is the concentration of acid?
1) Balanced eq
NaOH (aq) + HCl (aq) → NaCl (aq) + H2O (l)
2)moles of NaOH (known conc)
Moles NaOH = 0.10 x 30/1000 = 3.0 x 10-3 mol
3) Balanced equation: check mole ratio:
NaOH : HCl is 1:1
4) Amount (moles) HCl)
Moles = 3.0 x 10-3
5) Concentration of HCl
Conc. = 3.0 x 10-3/0.025= 0.12moldm-3
