Chemistry Syllabus Preliminary Thread (1 Viewer)

[pLuvia]

Same with biology ill try and help you with anything to do with the syllabus

 

[-neurosurgeon-]

me too lol

 

[velox]

get conquering chem and do the questions

 

[pLuvia]

do you understand avagadros number and the formulas that are used?

i.e.
n = moles
MW = molecular weght (sometimes referred to as molecular mass)
m = mass (grams)
N = number of atoms
Na = Avagadros number - 6.022 x 10^23

Formulas

n = m/MW

Example

How many moles are contained in each substance given below?

(a) 1.01 g of neon gas (Ne)
(b) 2.538 g of iodine molecules (I2)

(a) n = 1.01 g/20.18g/mol = 0.05 moles
(b) n = 2.538 g/253.80 g/mol = 0.01

n = N/Na

Example

For 3.011 x 10^23 atoms of C-12. How many moles do we have?

n = ?
N = 3.011 x 10^23
Na = 6.022 x 10^23

n = 3.011 x 10^23/6.022 x 10^23
n= 0.5 moles

Molar Volume of Gases

STP - Standard Temperature and Pressure
Temperature 0*
Pressure 101.3KPa
Vm 22.41 L/mol

SLP - standard Laboratory Condition
Temperature 25*
Pressure 101.3KPa
Vm 24.47L/mol

n = moles
V = Volume
Vm = Molar Volume

n = V / Vm

Example

Calculate the
(a) Number of moles
(b) mass of methane (CH4) in 5.0L of the gas at STP (Standard Temperature and Pressure)

(a)
n = V / Vm
n = 5.0L / 22.41L/mol
n = 0.22 moles

(b)
n = m / M
0.22 = m / 16.05g/mol
m = 3.53 g

Also remember these:
C = m/v (when calculating with mass per volume)
and/or
C = n/v (when calculating with moles per volume)

Where:
C = concentration (most common are g/L and mol/L or M)
n = number of moles
m = mass (g)
v = volume (L)

Examples:
1) Find the concentration of 1g of salt in 100mL of water in g/L.
2) What mass of calcium carbonate is required in 500mL of water to obtain a concentration of 36g/L?
3) What volume of water is needed to dilute 0.5mol sodium hydroxide to a concentration of 5M?
4) Find the MASS of calcium carbonate needed to form a concentration of 1M in 200mL of water.

Answers:
1)
C = m/v
.: C = 1/0.1 (i.e. 1 gram per 0.1L)
.: C = 10g/L
2)
C = m/v
36 = m/0.5
.: m = 18g
3)
C = n/v
5 = 0.5/v
.: v = 0.1L = 100mL
4)
C = n/v
1 = n/0.2
n = 0.2
BUT we need to find mass:
.: using n = m/M (i.e. moles = mass/molar mass)
M = CaCO3 ≈ 40 + 12 + (16 x 3) = 100
.: 0.2 = m/100
.: m = 20
.: mass = 20g

Ok, well I'll post some questions.
Post your answers, with working, and we'll be able to see where you're going wrong.

1. The molecular weight of caffeine is 194g. Caffeine is found to consist of 28.9% nitrogen.
Calculate the number of moles of nitrogen present in 534g of caffeine.

2. A solution of concentrated HCl contains 296g of hydrogen chloride dissolved in 500mL of water.
Calculate the percentage composition of the HCl.

3. In 20.0mL of 0.40 mol L-1 Al(NO3)3 solution calculate the following;
a)The concentration of Al3- and NO3-
b) The numbers of moles of Al3- and NO3-

4. What volume of 12.0 molL-1 HCl is required to prepare 500mL of 2.50 molL-1 solution?

 

[pLuvia]

hey, what topic you guys up to in chemistry, we're only up to hydrocarbons, i.e. how to name them, different allotropes

 

[Guernica]

Yep, we're doing hydrocarbons too

 

[Lavenderpup]

i've finished the course and its just revision for this whole week..but my exam is in 5 days give or take >_< and i haven't written any notes -___- geez, revision week and all but what about private study pft.

 

[Jimmy Pax]

We finished about a week ago, and have been going over the syllabus and just revising for preliminary tests next week.

 

[pLuvia]

you guys are so fast ><"...

 

[Necro_Nursey]

Hahaha we just finished hydrocarbons last week >.<
But tomorrow I have my prelim for chem
ahhh!
[jumps in the bath and hides]
its so scary
i dont know what we're doing next year tho, thats if i keep chem

 

[kazan]

can someone tell the the solubility rules, i lost my sheet and i cant rember them


22 hours till chem exam

 

[ta1g]

could someone explain gay-lussac law and avagrados law to me?

 

[pLuvia]

im not really sure about gay lussac's law but with avagadros law it is..

Na (avagadro's number) = 6.022 x 10^23

Formula

n = moles
N = number of atoms
Na = 6.022 x 10^23

n = N / Na

 

[ta1g]

thats avagrados number...i think his law is that with equal volumes of gases at the same temp and pressure have the same number of molecules or something like that and gaylussacs is equal volumes of gases at same temp and pressure can be represented in single whole number ratios..im not sure if that is right though.

 

[pLuvia]

don't think you need to know that

 

[kazan]

yes you do need to know it. it is

“Equal volumes of gas contain equal numbers of molecules at the same temp. and pressure.”


If equal volumes of gas contain equal numbers of molecules, and the volumes of gases taking part in a chemical reaction show simple whole number ratios to one another (all at constant temp. and pressure), then the numbers of molecules (or moles) taking part in a chemical reaction would also show simple whole number ratios to one another.

 

[Loz_metalhead]

avagadros law shows how many particles...

 

[[Damo]]

in my exam they asked for Gay-Lussacs law of volumes.
volumes of gasses can be expressed as small whole number ratios

 

[*yooneek*]

my chem xam is tomorrow!!!
dum dum dum!!! lol
it hasn't sunk in yet...
i need someone to slap me on the face and say
"oi you! finish your summary!"
im just so revved about holidays!!!
i have this vision of redoing my whole room into a study spectacular ready and raving for next term lol
xoxox

 

[pLuvia]

lol.... well it will still happen even if you dont study