Relative molecular mass is the average mass of one molecule of a substance compared to the mass of 1/12 of a carbon-12 atom.
Category: Pure Chemistry
Define relative atomic mass.
Relative atomic mass is the average mass of one atom of an element compared to the mass of 1/12 of a carbon-12 atom.
Write down the 3 formulas to find the number of moles of a substance.
No. of moles = Mass/ Mr or Mass/ Ar
No. of moles = Volume (dm3) x Concentration (mol/dm3)
No. of moles = Volume (dm3) / 24 dm3
Explain, using structure and bonding, why ethanol cannot conduct electricity.
Ethanol has a simple covalent structure. It exists as discrete molecules in the different states. There are no mobile ions or delocalised electrons to conduct electricity.
Why is there a need to recycle metals?
Metals are finite resources.
With the increasing demand for metals, our metal reserves will not last much longer.
Hence, there is a need to recycle metals to conserve the metal resources.
Recycling metals also use up less energy and produce fewer pollutants than extracting metals from their ores.
What are the observations when Group I metals react with cold water?
– Metal piece darts about quickly around the surface of the water
– Effervescence observed (due to hydrogen gas produced in water)
– Hissing sound heard (due to the vigorous release of hydrogen gas)
– Metal piece becomes smaller in size
– Flame observed (for some metals)
– Lithium (no flame)
– Sodium (yellow flame)
– Potassium (lilac flame)
– pH of solution increases from pH7 to pH14 (due to the formation of a strong alkali)
Explain, in terms of oxidation state, why the following reaction is a redox reaction.
PbS (s) + 4H2O2 (aq) -> PbSO4 (s) + 4H2O (l)
PbS has been oxidized to PbSO4.
The oxidation state of sulfur increased from -2 in PbS to +6 in PbSO4.
H2O2 has been reduced to H2O.
The oxidation state of oxygen decreased from -1 in H2O2 to -2 in H2O.
Since oxidation and reduction occurred simultaneously in the reaction, it is a redox reaction.
Format for explanation in terms of oxidation state
(The reactant substance) has been oxidized / reduced to (the product substance).
The oxidation state of (the element) increased / decreased from (o.s. no.) in (the reactant substance) to (o.s. no.) in (the product substance).
Examples:
Explain whether the underlined substance has been oxidized or reduced, in terms of oxidation state.
1. ZnO (s) + C (s) -> Zn (s) + CO (g)
Zinc oxide has been reduced to zinc.
The oxidatation state of zinc decreased from +2 in zinc oxide to 0 in zinc.
2. PbS (s) + 4H2O2 (aq) -> PbSO4 (s) + 4H2O (l)
Lead(II) sulfide has been oxidized to lead(II) sulfate.
The oxidation state of sulfur increased from -2 in lead(II) sulfide to +6 in lead(II) sulfate.
3. PbO (s) + H2 (g) -> Pb (s) + H2O (g)
Lead(II) oxide has been reduced to lead.
The oxidation state of lead decreased from +2 in lead(II) oxide to 0 in lead.
4. 2FeCl2 (aq) + Cl2 (g) -> 2FeCl3 (aq)
FeCl2 has been oxidized to FeCl3.
The oxidation state of iron increased from +2 in iron(II) chloride to +3 in iron(III) chloride.
Describe in terms of the arrangement and movement of particles, the process of ice melting.
When ice melts, the heat energy gained from surroundings is used to weaken the bonds between particles. The movement of the particles changes from vibrating in fixed positions in the solid state to moving and sliding over one another freely in confined spaces in the liquid state. The arrangement of the particles changes from regularly arranged, tightly packed together in the solid state to irregularly arranged, close together in the liquid state.
The average kinetic energy of the particles remains constant, so temperature remains constant during melting.
What is a buffer solution?
A buffer is an aqueous solution that can resist significant changes in pH levels upon the addition of a small amount of acid or alkali.
There are two types of buffer solutions: acidic buffer and alkaline buffer.
Acidic buffer
Acid buffer solutions have a pH less than 7. It is generally made from a weak acid and one of its salts. Commonly used acidic buffer solutions are a mixture of ethanoic acid (a weak acid) and sodium ethanoate in solution (a weak basic salt), which have a pH of 4.76 when mixed in equal molar concentrations.
Alkaline buffer
Alkaline buffer solutions have a pH greater than 7 and are made from a weak base and one of its salts. A very commonly used example of an alkaline buffer solution is a mixture of ammonia (weak alkali) and ammonium chloride solution (a weak acidic salt). If these were mixed in equal molar proportions, the solution would have a pH of 9.25.
How do buffers work?
Buffers work by neutralizing any added acid (H+ ions) or base (OH- ions), by producing a weak acid or a weak alkali, respectively, hence maintaining the required pH.
For example, a buffer made up of the weak base ammonia, NH3 and its conjugate acid, NH4+. When HCl (strong acid) is added to this buffer system, the extra H+ ions added to the system are consumed by the NH3 to form NH4+, which is a weak acid. Now, because all the extra H+ ions are locked up and have formed a weaker acid, NH4+, thus the pH of the system does not change significantly. Similarly, when NaOH (strong base) is added to this buffer system, the NH4+ ion donates a proton to the base to become ammonia and water, which is a weak alkali, thus neutralizing the base without any significant pH change.
H2O (l) + NH3 (g) ⇌ OH– (aq) + NH4+ (aq)
Example:
Answer is A. The mixture of ammonia and ammonium chloride solution acts as an alkaline buffer solution, maintaining the pH at about 10.