The concentration (unit: mol/dm3 or mol/l) is given by
c = n / V
where V is the volume (unit: dm3 or liter). Remember to convert everything into the same units.
3. What is the relative atomic mass ?
4. What is the relative molecular mass ?
5. What is the difference between relative molecular mass and molar mass ?
6. What is the formula with Avogadro's constant ?
7. What does the coefficients in chemical equations tell you ?
8. In the reaction 2A + 3B --> C one has 4 moles of C. How many moles of A is needed ?
9. What is the formula for molar mass ?
10. What is a chemical (or molecular) formula ?
11. What is the empirical formula ?
15. What is the mole ratio of X and Y in the reaction aX + bY --> cZ ?
16. How do you balance an equation ?
17. What are the symbols for different states ?
18. What about mass in a reaction ?
19. What is the limiting reagent
20. What is theoretical yield and percentage yield ?
21. How do one convert from Celsius to Kelvin degrees ?
22. What is the ideal gas equation ?
23. What is the molar volume of a gas ?
26. What is the formula for density ?
27. What is solvent and solute ?
28. What is the formula for concentration ?
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One Mole of something is equivalent to 6.023 x 1023 (Avogadro's constant) units of it (i.e. atoms, molecules etc). It is also defined as the number of atoms in exactly 12 grams of Carbon-12 isotopes. The number of moles is called n and it has the unit mol.
The molar mass (M) is the mass of one mole. Unit = g/mol .
The relative atomic mass (Ar) is the mass of an atom relative to one-twelfth the mass of an atom of carbon-12. It is basically equal to the number of protons and neutrons in the atom since the electrons have a negligible mass. If more than one isotope of the element is present, the relative atomic mass is calculated by taking an average that takes account of the relative proportions of each isotope, resulting in values that are not whole numbers. Unit = none.
The relative molecular mass (Mr) is the sum of the relative atomic masses of the atoms in the molecule.
Mr = M but the units are different. Mr do not have a unit but M has the unit g/mol.
n = N/NA
where n is the number of moles (Unit: mol), N is the number
of particles and NA is Avogadro's constant = 6.023 x 1023 .
The coefficients in chemical equations give the molar ratios of reactants and products.
For example, in 2A + 3B --> C, there is 2/3
as much A as B, and 3 times more B than C involved in the reaction. Assuming the reaction goes to completion,
there must be 3/2
times as much B as A for neither to remain. If this ratio is not
followed, one will be a limiting reactant, and so the reaction will have
some of the other reactant left over when it completes.
So one needs 8 moles of A to get 4 moles of C !
Given a molar mass (M) with unit g/mol, a mass (m) with unit g, and a
number of mols (n) with unit mol then
n=m/M
Moles is a number of something, sort of like a dozen. Every mol is 6.023x1023
individual elements. Mass is the property which results in 'weight' in the presence of gravity.
The chemical (or molecular) formula gives the true number of each atom in a molecule.
The empirical formula gives the ratio of the atoms in a molecule. Examples:
| Compound | Sulphuric acid
| Ethane
| hydrogen peroxide
| propane
| Ethene
|
| Molecular formula | H2SO4
| C2H6
| H2O2
| C3H8
| C2H4
|
| empirical formula | H2SO4
| CH3
| HO
| C3H8
| CH2
|
Determine the empirical formula for a molecule with 85.7% C and 14.3% of H.
Moles of C for 100g = m / M = 85.7 / 12.01 = 7.14
Moles of H for 100g = m / M = 14.3 / 1.01 = 14.16
The simples ratio is then C: 7.14 / 7.14 = 1 and H: 14.16 / 7.14 = 1.98
The empirical formula is CH2
What is the percentage by mass of nitrogen in NH4 ?
Mass of 1 mole of N is 14.01.
Mass of 1 mole of NH4 is 14.01 + 3 x 1.01 = 17.04.
Fraction of N in NH4 is then 14.01/17.04 = 0.82 or 82%.
Determine the molecular formula for a compound with empirical formula
CH2 and a relative molecular mass of 42.09
The empirical formula is CH2.
The molecular formula is CYH2Y
Mr = 12.01 x Y + 2Y x 1.101 = 14.03Y = 42.09 so Y=3
The molecular formula is C3H6
The mole ratio of two species in a chemical equation is the ratio of their coefficients (i.e. aX + bY --> cZ).
The ratio of X/Y is a/b, Y/Z = b/c etc.
Balancing equations: Change only the coefficients, not the subscripts, to make sure all atoms and all charges are conserved.
State symbols: (s)-solid , (l)-liquid, (g)-gas, (aq)-aqueous solution (something dissolved in water). These symbols should be included in all chemical reactions (but apparently you won't be penalised for leaving them out).
Mass is conserved throughout reactions. This fact allows masses to be calculated based on other masses in the reaction. For example, burn Mg in air to produce MgO to find the mass of Mg present in the original sample. This can be extended to concentrations.
When a reaction contains several reactants, some may be in excess (more is present that can be used in the reaction). The first reactant to run out is the limiting reagent (or reactant). Knowing the number of mols of the limiting reagent allows all other species to be calculated, and so the yield, and remaining quantities of other reactants.
The theoretical yield is the amount produced according to the equation.
The experimental yield is the amount that is actually produced.
The percentage yield = experimental yield / theoretical yield.
One goes from Celsius to Kelvin by adding 273 degrees.
p V = n R T
where p is the pressure (unit: Pa or N/m2), V is the volume (unit:m3)
n is the number of moles (unit: mol), T is the temperature (Unit: K) and R is gas constant
= 8.31 J/Kmol.
The molar volume of a gas (Vmol) is the volume of one mole.
n = V / Vmol
STP is Standard Temperature and Pressure: T = 273 K, p = 100 kPa.
RTP is Room Temperature and Pressure: T = 273+25 K, p = 100 kPa.
The density (unit: g/cm3 or kg/m3) is given by
ρ = m / V
where V is the volume and m is the mass. Remember to convert everything into the same units.
Solvent: The stuff you're dissolving in (i.e. water).
Solute: The stuff you're dissolving in it (an ionic compound or something)
Solution: The two of them when mixed together.
Concentration: The amount of solute per amount of solvent (in mols per dm3 (mols per liter or grams per liter).
The concentration (unit: mol/dm3 or mol/l) is given by
c = n / V
where V is the volume (unit: dm3 or liter). Remember to convert everything into the same units.