Flashcards in 1.2 Amount of substance Deck (17)
List the formulae you can use to find the no. of moles.
n=m/Mr moles=mass/molecular mass
n=cv moles=concentration (in moldm^-3) x volume (in dm^3)
What is the number of particles representing one mole?
"A compound is found to contain 23.3% magnesium, 30.7% sulfur and 46.0% oxygen. What is the empirical formula of this compound?"
Mg S O
23.3% 30.7% 46.0%
(Divide by Mr)
23.3/24.3 30.7/32.1 46.0/16.0
=0.959 =0.956 =2.875
(Divide by smallest value)
(/0.956) (/0.956) (/0.956)
= 1 =1 =3
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What is the ideal gas equation?
p = pressure in kPa
V = volume in m^3
n = moles of gas present
R = the gas constant (8.31 JK^-1mol^-1)
T = The gas' temperature in kelvin.
If a sample of boron is composed of 19.9% atoms with an Ar of 10, and 80.1% atoms with an Ar of 11, calculate the atomic weight of boron.
(19.9x10) + (80.1x11) / 100
= 10.8 (1dp)
How to remember which formulae to use:
FOR SOLUTIONS: n=cv
FOR SOLIDS: n=m/Mr
Calculate the volume of 0.50 moldm^-3 nitric acid required to react completely with 5 g of lead (II) carbonate.
PbCO3 + 2HNO3 --> Pb(NO3)2 + CO2 + H2O
Moles of PbCO3 = 5/267 = 0.0187
1:2 ratio- moles of HNO3 = 0.0187 x 2 = 0.0375
Volume of HNO3 = 0.0375/0.5 = 0.075dm^3
(0.075x1000=75cm^3 if answer needs to be in cm^3)
What's the formula for percentage yield?
actual yield/maximum theoretical yield x 100
What is the atom economy for making hydrogen by reacting coal with steam?
C(s) + 2H2O(g) → CO2(g) + 2H2(g)
Answer = 8.3%
12 + (2x18) → 44 + (2x2)
(2x2)/(12+36) x 100 = 8.3%
What is the formula for percentage uncertainty?
(Absolute uncertainty/Value) x100
What is the concentration of the solution if 2.5g of calcium carbonate is dissolved in water, and the solution made up to 0.5dm^3?
Answer = 0.0500moldm^-3
n=m/Mr = 2.5/40.1+12.0+(3x16.0) = 0.0250mol
c=n/v = 0.250/0.5 = 0.0500moldm-3
Calculate the Mr of a substance with a weight of 90g when there are 5 moles of it, and suggest what the substance could be.
Answers = 18, water.
Mr = m/n = 90/5 = 18
Water has an Mr of 18.
Deduce the formula of the compound lead (IV) oxide.
What is the mass of sulphur in 1 tonne of H2SO4?
Answer = 327kg
1 mole of H2SO4 = 98.1g
1000000 grams in a tonne
No. of moles in a tonne = 1000000/98.1 = 10200mol
10200 x 32.1 = 327000g (327kg)
25cm^3 of a solution of 0.1moldm^-3 NaOH reacts with 50cm^3 of a solution of hydrochloric acid. What is the molarity of the acid?
Answer = 0.05moldm^-3
NaOH + HCl → NaCl + H2O
Moles of NaOH: n=cv = 0.1 x (25/1000) = 0.0025mol
∴ There are 0.0025 moles of HCl
c=n/v = 0.0025/(50/1000) = 0.05moldm^-3
How many moles of NaCl are there in 25cm^3 of a 50gdm^-3 solution?
n=m/Mr = 50 / 23.0+35.5 = 0.855moldm^-3
n=cv = 0.855 x (25/1000) = 0.0214mol