Amount Of Substance Flashcards Preview

AMOUNT OF SUBSTANCE > Amount Of Substance > Flashcards

Flashcards in Amount Of Substance Deck (60)
Loading flashcards...

How many particles does one mole contain ?

6.02 x 10^23


Formula for converting between no. Of particles and no. Of moles

Number of particles = number of moles x avagadro’s constant


What is the moles , mass and mr equation

Moles = mass/mr


Important conversions

1 ton = 1000000g
1kg = 1000g (x 1000)
1mg = 0.001g ( divide by 1000)


What are the three steps to moles mass and mr equation

Step 1 : find number of moles of the compound which the mass is given for
Step 2 : determine ratio of number of moles between the compound which we had the mass for and the compound that we need to find the mass for
Step 3 : find the mass of the compound using the number of moles
Mass = mr x mole


What are the steps to a moles, mass and mr question when we are given the mass of both reactants

Step 1 : calculate the number of moles for each compound (eg 3.75 and 0.35)
Step 2 : pick one compound and find the ratio of the other to see which one is in excess and which one is limiting
(Eg if 0.35 and ratio 1:2 then we only need 0.70 but we have 3.75)
The limiting one is the one we use
Find the ratio of the limiting reactant and the product we need to find
Step 3 : find the mass of the product


What formula do you use to calculate no. Of moles from concentration ?

Moles = concentration x volume


What is concentration equation

Conc = moles/vol


Important conversions for concentration equation

Cm 3 to dm3 = divide by 1000
Dm 3 to m 3 = divide by 1000
Cm 3 to m3 = divide by 1000000


Steps to calculating normal concentration equation

Step 1 : Use equation to find number of moles of given reactant
Using the concentration of it and the volume
Step 2: use ratio to find number of moles of next reactant for product
Step 3 : find conc using equation


Steps to find concentration when given the mass of a reactant

Step 1 First use moles = mass/ mr for the given reactants
Step 2 : ratio to find number of moles for other compound
Step 3 : find conc


How do you use the concentration equation in a question where you are given the moles of one reactant and the vol and conc of reactant and you need to find the product

Step 1 : find the number of moles of the one where you are given the concentration and the volume
Step 2 : find the ratio between this and the compound with the number of moles given.
Find out which one is in excess and which one is limiting
The limiting one should be used in the ratio with the compound you are trying to find


ZnO + 2HCl -> ZnCl2 + H20
ZnO = 0.0830 moles
2HCl = 100cm3 and 1.20 mol dm-3
Find the mass of ZnCl2

Step 1 : find moles of both
Already given moles of ZnO
Moles of HCl = conc x vol = 1.20 x 100/1000=0.12
Step 2 : ratio of HCl to ZnO
2:1 so 0.12/2 = 0.06 0.06 is less than ZnO so HCl limiting
Step 2.5 : ratio of HCl and ZnCl2 = 2:1
Moles of ZnCl2 = 0.06
Step 3 : mass = mr x mol


What is the molecular formula ?

The average masses of atoms in formula compared to 1/12th mass of an atom of carbon 12


What is a standard (volumetric ) solution ?

A solution that you know the exact concentration of


What does the making of a standard solution involve ?

Involves dissolving a known amount of solid in known amount of water to create known concentration molecules


Describe how would you make 250 cm3 of 2.00 mol dm-3 solution

1. 0.5 moles of sodium hydroxide needed and 20.0g of sodium hydroxide needed weighing boat on digital balance and weigh out mass of solid
3. Tip it into beaker
4.reweigh boat
3. Subtract mass of boat from mass of boat and solid together = find precise mass of solid. (Record difference).
4. Add distilled water to beaker (eg 100dm3) and stir until sodium hydroxide dissolved
5. Pour solution into 250 cm3 volumetric flask using funnel
6. Rinse beaker , stirring rod with distilled water and add washing to flask
7. Add solvent to correct volume until graduation line then drop by drop until bottom of meniscus reaches the line.
8. Invert flask several times to make sure it is mixed and a uniform solution


Why do you rinse the beaker and stirring rod with distilled water and add that to flask

Makes sure no solute clings to the beaker or rod


What do titrations allow you to do ?

Allow to find out exactly the volume of acid needed to neutralise quantity of alkali


What is the method for titration

1. Use Pipette to measure 250 cm3 set volume of solution/ solid you want to find concentration of into conical flask
2.add few drops of indicator
3. Fill Burette with standard solution of acid HCl) mention what concentration the HCL or acid is
4. Use funnel to pour acid into burette - done at eye level
5. Do rough titration to get idea of end point by adding acid to alkali using burette and give flasks regular swirls
6. Do accurate titaration
Take initial reading of acid
Run acid with 2cm3 of end point
Add acid drop wise until end point reached


How do you find volume of titre

Final reading - initial reading


Until when should you repeat the titration

Until you have 3 concordat results


What is a anomalous result

A result that it more or less than 5 away from the other results


What kind of indicators should be used ?

Ones that change colour quickly so have a small ph range so we know exactly when the reaction has ended


Why is it important to wash/ rinse the inside of flask and why does it not affect the titre value

So that all the solution reacts with each other and is in the flask
Water does not change number of moles in either reactant


Why is it essential to remove the funnel after being used to fill the burette

Droplet from funnel could enter the burette and effect the readings


What is the source of error in using a burette

Air bubbles


What is the effect of the air bubble

The value of the titre would be higher


Why is a conical flask used rather than beaker

Less chance of losing liquid in swirling


How do you calculate % uncertainty

Error/ measurement x 100


What two things do you have to be careful about when it comes to uncertainty ?

1. How much the equipment has been used (if twice then error x 2)
2. Make sure it is a measurement not a reading ( might have to find the difference in readings for the measurement)


How can you reduce uncertainty ?

Reducing the error (eg more accurate balance or apparatus with greater resolution)
Decrease concentration of solution in burette to increase volume measurement


What do burettes measure and let you do ?

Measure different volumes and lets you add solution drop by drop


What do pipettes measure

Only one volume of solution


If there is only two results what should you do ?

Do further titrations for concordant results


How is the method for standard solution accurate

1. The precise mass of solid is obtained by subtracting mass of boat from mass of boat and solid together
2. Wash the rod and add washings to the beaker
3. Ensure bottom of meniscus reaches is on graduation Lind


What is the ideal gas equation



What are all the units of the ideal gas equation

P= pressure (Pa)
V= volume (m3)
N= number of moles
T= temperature (k)


Dm3 to m3
Cm3 to m3

Dm3 to m3 = divide by 1000
Cm3 to m3 = divide by 1000000


Pressure :
Kpa to pa
Map to pa

Kpa- pa X 1000
Mpa- pa X100000
Atmosphere - x101325


How do you convert Celsius to K

C + 273


What is the rule for volume of gases

At room temperature and pressure 1 mol of any gas occupies 24 dm3


How do you find the mass of single atom in a element

Mr/ Avogadro constant


Ideal gas equation rearranged

For number of moles = Pv/rt
Temperature = pv/ nr
Volume = nRt/p
Pressure = nRt/V


What is the formula for atom economy

Mass of wanted product from equation / total mass


What is the formula for percentage yield

Mass of product formed / maximum theoretical mass of product


Why do companies in the chemical industry try to use processes with high atom economies

Better for environment so they produce less waste
More efficient use of raw materials so more sustainable
Less expensive


In a titration a burette with an uncertainty of 0.05cm3 is used. Initial reading = 50cm3. The final reading 28.8cm3

Two burette readings = 0.05 + 0.05 = 0.01cm3
0.1/21.2 x 100= 0.5 %


Why do we re weigh the boat ?

It may contain traces of solid


If a question already has the yield given how do you answer the question (theoretical mass ) step by step

Step 1. Find number of moles of reactant.
Step 2. Ratio to find number of moles of product
Step 3 number of moles is the certain yield for example 62% but we need the theoretical mass (100%)
Step 4: divide number of moles by percentage then multiply by 100
Step 5: mass


How do you find the mass of the percentage yield when just given the percentage and mass of reactant but not the mass of percentage
Normal question : 510g actually made
This question : 90% percentage yield

Step 1: find moles
Step 2 : ratio
Step 3: mass
This mass is the 100% so we need the specific percentage so mass should be multiplied by the percentage yield needed/100


If a question asks you to do a titration of a reaction but has not given a certain mass of a solid or volume of and concentration of acid what should you do

For mass : moles, ratio mass
Moles would be found through concentration equation
For volume and concentration = rearrange concentration equation to find what you are looking for


How do you find the molecular mass from formula mass and empirical formula

Step 1 : write down empirical formula and mr (CH2)(mr=42)
Step 2 : find the formula mass which is just adding the Ars together Fm -12+2=14
Step 3 : find multiplier by dividing mr given by formula mass 42/14=3
Step 4 : multiplier x empirical formula 3xCH2=C3H6


How do you find the empirical formula

1) write out each element
2)write all masses or mass percentages of each element
3)divide mass/% by ar
4) divide smallest number by every other number to give whole number ratio


How do you find a mass of certain element if just given compound for example CO2 (just want to find carbon)

First find total mass of compound = should always be given!!
Do mr of element divided by total mr of compound and multiply by total mass to give percentage of that specific element
Eg mass = 1.10g
12/44 x 1.10 = 0.3
12 = mr of just carbon 44= mr of co2
0.3g of carbon


What should you be careful about when calculating empirical formula of a compound with Same element reacting with different things

The empirical formula of the certain element is split between every time that element reacts with something
Eg if it says (nh3 )3 at the start but (nh4)x at the end and empirical formula of n is 7
There is already 3 ns being used up so the x would be - 7-3=4 x=4


How do you convert from mol dm3 to g dm3

Multipliy by mr


How can you check if all the water has been removed

Reheat until mass hasn’t changed


Empirical formula question

1. Record mass of dish
2. Add known mass of hydrated metal Sulfate
3.heat over a Bunsen burner until mass has reached a constant mass - meaning all the water has been lost and the mass is not changing
4. Record mass of evaporating dish and contents and use this to calculate the mass of the anhydrous salt and the mass of water lost
5. Divide by mr and then smallest


How do you you find out the mass of the water and solid ?

mass of evaporating dish and solid before heating - mass of empty evaporating basin = about of solid before heating
Mass of evaporating basin before heating - mass of evaporating basin and solid after heating = amount of water
Amount of solid before heating - amount of water = amount of solid after heating