Note: the C below means degree celcius

Skill 1 : Calculating Energy

A 2kg copper object is heated from 30 C to 35 C. How much increase in thermal energy is required, given that the specific heat capacity of copper is 400J/Kg K

first step, use the heat capacity as your guide. write it as 400J/ 1Kg 1Know the qns asked for 2kg, so simply multiply both side by 2

400J / 1Kg 1K[when 1side has be multiplied by 2, another side will be the same]800J / 2Kg 1K

Note: please do not multiply by 2 such that it became 2Kg 2K, this means you are actually multiplying by 4! it works like this, 2 x 2 = 4 see the link?

now the next time is the find the temperature difference, 35 - 30 = 5

so do the same thing, multiply both sides by 5

4000J / 2Kg 5K

noticed that the 2kg remains unchange.

so thats your ans, you need 4000J !

Skill 2 : Calculating mass

4000J of energy is used to heat up a copper block from 50 C to 70 C. What is the mass of the copper block, given that the specific heat capacity of copper is 400J / Kg K?

this part is a little bit different, its about finding the mass but just do the same thing, write it down first

400J / 1Kg 1K

it mention 20 C so lets just multiply by 20

8000J / 1Kg 20k

now you have 8000J, its way of course of wat the question is asking which is only 4000J

so now jus bring it down, by dividing it by 2, however, u cannot divide temperature change as the qns said that it increases by 20 C. so simple, jus divide Kg by 2

4000J/0.5Kg 20K

hence, the answer is 0.5kg. easy?

Skill 3 : Calculating temperature change

4000J of energy is used to heat up a 5Kg copper block from an initial temperature of 20 C. What is the final temperature of the copper block, given that the specific heat capacity of copper is 400J / Kg K

400J / 1Kg 1K

copper is 5Kg, so multiply by 5

2000J / 5Kg 1K

u need 4000J so to change from 2000J to 4000J you have to muliply 2 again, Kg should not be change

4000J / 5Kg 2K

Hence, it raises 2 K, final temperature is 22 C

thats half of capacity covered. Simple eh?

time for latent heat

no need formula, how calculate latent heat etc. its the same method as heat capacity

wats more, there isn't any temperature to calculate! well but i think i better go through 1 last time

Eg: what is the amount of energy needed to melt 10kg of ice when the specific latent heat of fusion is 336000J/Kg

To get 10kg, multiply by both sides, 3 360 000J/10Kg so your ans will be 3360000

Another type of question that you have to learn how to tackle

what is the total energy required to heat 2kg of pure ice at -5C to 10C of water?

Specific heat capacity of ice : 2100J/Kg K

Specific heat capacity of water : 4200J/Kg K

Specific latent heat of fusion of ice : 336 000J/Kg

This question may seems a complicated question but it is still only by one simple concept. This process has three stages. As everyone know, every substnaces and states have different specific heat capacity hence, we cannot directly take the temperature change as 15. In this question, there are 3 things you have to find.

1. Amount of energy to raise the temperature of ice

2. Amount of energy to melt

3. Amount of energy to raise the temperature of water

why do i have to split it up? this is concern about different amount energy in different states, thats is y, total of 3 different heat capacity/latent heat has to be used

1st, you have to calculate the amount of energy needed in the first stage, raising temperature of the ice. The maximum temperature of ice can reach is of coz 0C as it has to undergo the change of state from solid to liquid now

1st stage:

2100J/Kg K --> x2 each side for its mass

4200J/2Kg K --> x5 for each every K change

21000J/2Kg 5K --> the ice has now reached 0C

Therefore, energy needed = 21000 in the 1st stage

2nd stage:

since its changing state, you have to use latent heat now

336 000J/Kg ---> x2 for mass

672000J/2Kg

Therefore, energy needed to melt the ice = 672 000J

3rd stage:

Now it has changed into water, bear this in mind, the water is still at 0C

4200J/Kg K ---> x2 for mass again

8400J/2Kg K ---> x10 for temperature

84000J/2Kg 10K

Therefore, energy needed in this stage is 84000J

now final step, ADD EVERYTHING TOGETHER FOR TOTAL ENERGY!

84000J + 672000J + 21000J = 777 000J (Answer)

you hafta practise more urself :D

FAQ: why do we still have to multiply the mass again when we had alr multiply it in the first place? the mass never change wat

A: thats y i say, heat capacity is only base on 1 concept, when 2kg of ice melted, there is still 2kg of water remains! the same thing has to be repeated

FAQ: What about Watts leh?

A: take the last qns for eg: how long does it take for a heater wif 200W to complete this process?

200W can jus simply covert it to 200J/s so you can do proportion easier

back to the qns, since it is 200J/s ask primary sch qus to urself, how many '200' do i need to reach 777 000J? so jus simply 777 000 divide it by 200 and tada! ans is 777 000 / 200 = 3885seconds!

last part of heat capacity

this is recommanded not to know wats the formula is talking about. understand it when ur heat capacity is very stable. so at the moment, jus study it w/o knowing wats happening

the formula is : mc(temp change) = mc(temp change)

m = mass, c = specific heat capacity

this formula is used when 2 substances are mixed together at different temperature and will give u a final temperature

Example

2kg of copper ball at 120C is thrown in 10Kg of water at 40C. Assuming that no heat is lost to the surrounding, what is the final temperature of the mixture?

Specific heat capacity of copper = 400J/Kg K

Specific heat capacity of water = 4000J/Kg K

lets take copper for the left side and water at the right side

X is the final temperature

(2)(400)(X-120) = (10)(4000)(40-X)

NOTE!!!: the temperature change part, cannot be in the same sequence means when x is on the left side of the bracket, the another side of the eqn CAN NOT hav X on the left side of its bracket too

800(x-120) = 40000(40-x)

800x-96000 = 1600000 - 40000x

x = 41.6C

thats the ans, if you were given final temp and find mass of specific heat capacity, jus put the final temperature into X and find the other unknown

anything gotta do wif finaly temperature can use this formula to solve

THE END! PRACTISE AND WORK HARD!

try this

problem example 1

What is the ratio of the volume of 2g of hydrogen to the volume of 16g of methane, both volumes at r.t.p?

when you give up / check answer pls highlight the space below :)

Follow the step, find mole first.

mol of hydrogen : 2g/2 = 1mol=1mol

Note: hydrogen is exist as H2 when left alone, otherwise in ion form]

mol of methane(CH4) : 16g/16 = 1mol

now, apply the volume of gas formula : mol x 24dm^3

so 1 x 24dm^3 = 24dm^3

Vol. of Hydrogen : Vol. of Methane

..........24.............:...........24

...........1..............:............1 ( answer )

easy? jus keep applying that 4 basic formulae and you have mastered 40% of mol concept. Fast? LOL now next i will teach you how to tackle another type of question which is about pencentage mass

heres the basic concept between the relationship between Ar / Mr and mass

Mr = total number of neutrons + protons

Since neutrons and protons are weigh same, it does not matter if neutrons are more than protons. imagine Mr is the same as mass but DO NOT use it as mass! [Eg: a 56Ar of Fe does not means Fe has 56g, i give u 1 bar of iron and it weigh 56g? LOL not logic right?]

The Mr only provide us the UNITS confused? don worry... now the method im going to teach is PROPORTION METHOD! this method in fact works with concentration, heat capacity, air pressure and waves

problem example 2:

Calculate the mass of water in 80g of MgSO4.7H2O

The 'dot' means the number of water molecules surrounding the compound. Hence, in this case, there are 7 water molecules surround 1 MgSO4 molecules. Clear?

The crystal weigh 80g, and since Mr is the CAUSE of weight so...

Find the Mr of the compound :

24 + 32 + 4(16) + 7(18) = 246

now, take it as units

246units ---> 80g

your finding mass of water so its Mr is : 7(18) = 126

so basic maths, you are finding 126 units, so you have to find 1unit first

1unit ---> 80g / 246

126units ---> 80g/246 x 126 = 41g

simple?

Now try this:

Calculate the mass of zinc in 48g of zinc carbonate

Mr of ZnCO3 = 125

125 = 48g

Ar of Zn: 65

Mass of Zn: 65 / 125 x 48 = 25g

Using proportion method on concentration: 0.1mol in 50cm^3

1st, put it together as 0.1mol/50cm^3

change the 50cm^3 to 1dm^3 which is, you have to times by 200

when one side times by 200, the other side must also times by the same no. that is, 200

hence, 0.1 x 20/ 50 x 20 = 2mol/dm^3

now done, lets proceed to, in tackling equations question that everyone hates ^^

heres the steps for eqn problem solving

1st step: find the mol ratio

2nd step: use the mol ratio to find the mol using any information given and the 4formulae

3rd step: using proportion method, mol ratio provide UNITS, use it to find the mol of another substance

4th step: after getting the mol of the substance you want, use the 4basic formulae again to find whatever the question asked

However, there are difficult eqn qns that will involves limiting reagent, percentage yield and molecular formula. now, i will demostrate 1 simple equation but you will have to REALLY DO MORE on 10yrs series or other assesment books

Limiting reagent is the substances that is LACKING in the reaction which means, this substance will use up first

Percentage yield is nothing to be terrified, all you have to do is to find mol, then find mass. its formula is Actual Yield / Theoretical yield x 100%. blur? not a problem, just remember, the 2 mass you have gotten, the smaller will ALWAYS divided by the larger number. forgot? use common sense, can u get more than 100%? :D no~ right? so its simple. the actual yield is given in the question most of the time.

try this qns, i know its difficult but see how far can you do first b4 u start to highlighting it :P

problem example 3:

2H2 + O2 ---> 2H2O

Given that 72dm^3 of hydrogen react with 48dm^3 of oxygen

(a) Which is the limiting reagent?

(b) Find the volume of oxygen reacted?

(c) Find the mass of water produced?

(d) What is the total number of water molecules formed?

(e) What is the percentage mass of oxygen in water?

(f) Find the percentage yield when only 60g of water produced

(g) Find the concentration of the solution if there is 21g of salt[NaF] dissoved into 500cm^3 of water

Solution:

a) Hydrogen , when 72dm^3 of hydrogen is reacted

b) 36dm^3

mol of hydrogen = 72/24 = 3

using mol ratio

Hydrogen : Oxygen = 2:1

so mol of oxygen is 3/2 = 1.5

Volume of oxygen : 24dm^3 x 1.5 = 36dm^3

remember...mol ratio are only units

c)since hydrogen and water have the same mol ratio, they have the same mol which is 3

using the formula: 3 x 18 = 54g

d) once again using the formula: 3 x 6 x 10^23 = 1.8 x 10^24

e) proportion method again: 16/18 x 100% = 88.9%

f) for concentration, you have to find the amount of substances dissolved hence since concentration is mol/dm^3 you have to find the mol of NaF

mol of NaF: 21/42 = 0.5

now, put everything together again: 0.5mol/0.5dm^3 = both sides x2 = 1mol/1dm^3g) always the smaller divided by larger...so..

percentage yield : 54/60 x 100% = 90%

ok last question on molecular formula

In a reaction, 4g of carbon reacts with 0.6g of hydrogen. The compound formed has a molecular mass 28g

(a) Determine the emptical formula for the compound

(b) Determine the molecular formula for the compound

Solution:

(a) due to presentation expectation from koh yilin =.= we have to draw table zzz

as usual, always find mol first

....................C.........H

Mass given.. 4......... 0.6

Ar is ...........12......... 1

Mol........... 0.3........ 0.6

hence, through mol ratio that you have jus find out, it is the no. of atom with different element in the formula [ in this case since carbon to hydrogen is 1:2 so the formula is CH2] simple?

b) molecular formula is the easiest, just divide the Mr of the molcular formula compound (most of the time is given in the question) by the Mr of the empirical formula compound.

Mr of CH2 = 14

Mr of molecular=28

divide it, 28 / 14 = 2

this number that u get, MULTIPLE it into the formula

(CH2)2 == C2H4

NOTE: PLS DO NOT WRITE 2CH2 ITS WRONG!

this is reEDITed version, i've found out why every1 is confused at concentration le~

concentration is the amount of SOLUTE dissolved in the solution, hence you can 'split' the concentration into parts for you to get the informations

its like dissolving 50g of solute [eg: NaCl] into 100cm^3 of water so your concentration can be written as 50g/100cm^3 however, most of the time we leave it in mol/dm^3 form. the method is simple

by proportion method, you have to change the 100cm^3 into 1dm^3, therefore, u simply jus multply by 10 to get 1000cm^3=1dm^3

now ur current concentration is written as 500g/dm^3 so how to convert it into mol/dm^3? this will be jus another basic, simply view the 500g separately. using 1 of the 4basic formula, convert mass into mol, 500/58.5 = 8.54mol so now the concentration is 8.54mol/dm^3 !

Note: you can always view the mol or mass part separately from the concentration BUT BUT BUT! ALWAYS, take note of the volume, if its 50cm^3 you have to divide it by 200 from 1dm^3

Titration formula: n1/n2 = m1v1/m2v2

n= mol ratio, m=mol,v=volume

this is another method that can use to find the unknown but ONLY in titration

thats all for mol...u have to do quite a lot urself or else its not easy to master

Identification: any front can go with any back in the formula ignore acid

Neutral oxide

Front - Back

Non-metal + O

[only 1 oxygen plus a non-metal will be neutral oxide]

{Eg: CO, SO, H2O}