chem calculation help (1 Viewer)

[liverpoolfc]

I got some calculations i am having trouble with to do with moles and concentrations etc..

Does any1 have any example questions (simple ones) to get me on the right track.

 

[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

Hope this helps you. Good luck~!

 

[A l]

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

 

[Dreamerish*~]

I don't have any questions at hand. It's too late and I'm delirious.

Go bug your teachers, they'll give you some practice questions. If not, raid your textbooks for calculations questions.

If not, come back here tomorrow and I'll have some posted.

 

[liverpoolfc]

what does 1M mean? Does it mean 1 mole per litre?

 

[Dreamerish*~]

what does 1M mean? Does it mean 1 mole per litre?

Yes. Yes it does.

 

[Emma-Jayde]

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 Al^3- and NO₃^-
b) The numbers of moles of Al^3- and NO₃^-

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