Chemistry MCQs

Notes:
The atomic mass (A) of Uranium (92U238) = 238; the number of protons (p) = 92 = number of electrons (e). Since atomic mass (A) = p + n (number of neutrons), A = 238 = 92 + n. So, n = 238 -92 = 146

Notes:
According to the Collision Theory, a chemical reaction can occur only when the reactant molecules, atoms, or ions collide with more than a certain amount of kinetic energy and in the proper orientation. In order to effectively initiate a reaction, the reactants must be moving fast enough (with enough kinetic energy) so that they collide with sufficient force for bonds to break. This is known as the activation energy. However, even if two molecules collide with sufficient activation energy, there is no guarantee that the collision will be successful. In fact, not every collision is successful, even if molecules are moving with enough energy. The reason for this is because molecules also need to collide with the right orientation, so that the proper atoms line up with one another, and bonds can break and re-form in the necessary fashion. An effective collision is defined as one in which molecules collide with sufficient energy and proper orientation, so that a reaction occurs. For example, in the gas-phase reaction of dinitrogen oxide with nitric oxide, the oxygen end of N2O must hit the nitrogen end of NO; if either molecule is not lined up correctly, no reaction will occur upon their collision, regardless of how much energy they have.

Notes:
Mass of 4 moles of aluminium atoms = 4 × molecular mass of aluminum atoms in grams = 4 × 27 = 108

Notes:
Ice is not a mixture because the atoms in the ice are locked in a crystalline matrix whose atoms are less densely packed than those in the liquid water, a unique property of water.

Notes:
The law of definite proportion, sometimes called Proust’s law, states that a given chemical compound always contains its component elements in fixed ratio (by mass) and does not depend on its source and method of preparation. For example, oxygen makes up about 8/9 of the mass of any sample of pure water, while hydrogen makes up the remaining 1/9 of the mass.

Notes:
A high-octane-rated fuel, such gasoline contains lead. Straight-run gasoline is distilled directly from crude oil. Once the leading source of fuel, its low octane rating required lead additives. Most countries have phased out leaded fuel. Different additives have replaced the lead compounds. Such as aromatic hydrocarbons, ethers and alcohol (usually ethanol or methanol).

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The main gases responsible for causing acid rain are sulphur dioxide as well as nitrous oxides. The main sources of sulphur dioxide are metal working industries and coal-fired power stations whereas nitrous oxide is from vehicles and fuel combustion.

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Chlorine gas is a component of tear gas, also known as Chloroprene. The most commonly used tear gas contains the chemical agent 2- chlorobenzaldene malononitrile (CS). Despite its name, tear gas is not a gas, but an aerosol. CS is solid at room temperature and mixed with liquid or gas dispersal agents when used as a weapon designed to activate pain-sensing nerves.

Notes:
Silica, or more correctly silicon dioxide (SiO2), is one of the most abundant compounds on the surface of the Earth. Silica has a multitude of uses depending on the degree of purity. Grades up to 99.5% SiO2 are used in the manufacture of glass, optical fibres and ceramics. It is also used in solar panels and integrated circuits used in computers.

Notes:
The density (in kg/m3) of given metals is as follows : Lithium : 534; Vanadium : 5494; Iron : 7850; Gold : 19320. Under standard conditions, Lithium is the least dense of all elements that are solids at room temperature. Lithium can float on the lightest hydrocarbon oils and is one of only three metals that can float on water, the other two being sodium and potassium.