Sindh Mdcat Exclusive Course Chemistry Intro To Fundamental Concepts — Solved MCQs

Question 1

Many elements have fractional atomic masses. This is because:

Accepted Answer

D. Atomic masses are average masses of isotopes proportional to their abundance — Atomic masses are typically reported as weighted averages that take into account the abundance of each isotope of an element. Most elements exist as a mixture of isotopes, and these isotopes have different masses. The atomic mass of an element listed on the periodic table is a weighted average of the masses of its naturally occurring isotopes. The weighting is based on the abundance of each isotope in nature. Since isotopes have different masses, the atomic mass is not a whole number, and it may appear as a fractional value.

Question 2

In which of the following will water evaporate earlier?

Accepted Answer

B. Saucepan — The rate of water evaporation depends on various factors such as the surface area exposed to air, temperature, humidity, and airflow. In general, water will evaporate more quickly in conditions where these factors promote evaporation. If we assume that the initial conditions (temperature, humidity, etc.) are the same for all containers, then the primary factor influencing the rate of evaporation would be the surface area exposed to air. Larger surface areas allow for more water molecules to be in contact with the air, promoting faster evaporation. A wide saucepan typically has a larger surface area compared to a cup, glass, or small bowl. Therefore, under similar conditions, water is likely to evaporate earlier from a saucepan than from the other containers.

Question 3

Different kinds of atoms of the same element are called isotopes having different _ but same _ properties.

Accepted Answer

B. Physical, chemical — Different kinds of atoms of the same element, which have the same number of protons but different numbers of neutrons (resulting in different masses), are called isotopes. Isotopes of the same element have different physical properties, such as different atomic masses and, in some cases, different nuclear stabilities. However, isotopes of the same element have the same chemical properties. This is because chemical properties are primarily determined by the number and arrangement of electrons, which is determined by the number of protons in the nucleus. Since isotopes of the same element have the same number of protons, they exhibit the same chemical behavior.

Question 4

The unit used to express the relative atomic mass is called?

Accepted Answer

D. Atomic mass unit — One atomic mass unit (1amu) is a mass unit equal to exactly one-twelfth (1/12th) the mass of one atom of carbon-12 isotope, which is the scale used to compare atomic masses. So, the unit used to express relative atomic mass is amu. Thus, option D is correct.

Question 5

Which of the following has six isotopes?

Accepted Answer

C. Calcium — Calcium has six stable isotopes: 40, 42, 43, 44, 46, 48Manganese has four stable isotopes: 52, 53, 54, 55Tin has ten isotopes: 112, 114, 115, 116, 117, 118, 119, 120, 122, 124Carbon has three isotopes: 12, 13, 14.So option C is correct.

Question 6

How many molecules are there in 2.1 moles of CO2:

Accepted Answer

C. 1.26 x 10²⁴ — Since there are 2.1 moles of CO₂, we can easily find out the number of molecules present in it by multiplying 2.1 with the Avagadro's constant, which is 6.02 × 10²³. Hence, the answer would be approximately 1.2 x 10²⁴. The answer choice that is closest to the obtained value is 1.26 x 10²⁴. Please do not calculate the total number of atoms in this question; you have only been asked to calculate the total number of molecules.

Question 7

Avogadro's constant is the number of:

Accepted Answer

B. Atoms in 24g of Mg — Avogadro's Number (NA) is the number of atoms, molecules, ions, or anything in one mole of the respective substance. As 6.02 x 1023 atoms, molecules, ions, or electrons are present in one mole; therefore, this number is known as Avogadro's number.Only 24g of Mg equals one mole of Mg. Thus, only 24g of Mg has Avogadro's number of atoms.1g of He equals 1/2 mole of He. Thus it has half the number of atoms than Avogadro's number.35.5g of chlorine equals 1/2 mole of chlorine gas. Thus, it has half the number of molecules than Avogadro's number.2g of H equals two moles. Each mole of H atoms has 1 mole of electrons only, so two moles of H have twice the number of electrons than Avogadro's number.

Question 8

Find the mass in grams of 1.69 mol of phosphoric acid:

Accepted Answer

A. 165.6 g — Moles = Mass / molar mass Moles x molar mass = mass in grams Molar mass of phosphoric acid H₃PO₄ = 98 g / mol 1.69 mol x 98 g /mol = mass in grams Mass = 165.6 g

Question 9

Natural gas in homes burns in excess of oxygen, what is the limiting reactant in this case?

Accepted Answer

B. Natural gas — The limiting reactant is the substance that gets completely consumed first, stopping the reaction from continuing. Since oxygen is in excess, the natural gas (primarily methane, CH₄) is the limiting reactant because it determines how much combustion occurs.

Question 10

Spectrometry is used when reactants and products absorb _.

Accepted Answer

D. All of these — Spectroscopy is a method of understanding molecules by measuring the interaction of light and matter. By analyzing the amount of light absorbed or emitted by a sample, we can determine the sample's components, characteristics and volume.It can absorb UV radiations, visible radiations and infrared radiations.

Question 11

Formula mass is considered for _ compounds instead of their molecular mass:

Accepted Answer

B. Ionic — The correct answer is "Ionic" Formula mass is considered for compounds instead of molecular mass for ionic compounds.

Question 12

Molecular ions are produced by passing a high-energy electron beam or an x-ray beam through:

Accepted Answer

C. Gas — The ion that is produced when a molecule loses or gains one or more electrons is called a molecular ion, e.g., CH4+, CO+, or N2+. Cationic molecular ions are more abundant than anionic molecular ions. When a high-energy electron beam or an X-ray beam passes through a gas, it can produce molecular ions. The high-energy particles, either electrons or X-ray photons, interact with the gas molecules, resulting in ionization. In the case of electron ionization, a high-energy electron beam is directed at a gaseous sample. The electrons collide with the gas molecules, leading to the removal of one or more electrons from the molecules.

Question 13

A compound has an empirical formula, C2H2O. If the experimental molecular weight is found to be in the range of 160–170, the molecular formula of this compound is: (Atomic weight: C = 12.1, H = 1, O = 16).

Accepted Answer

C. C8H8O4 — The molecular mass of the empirical formula is 2(12.1) + 2(1) + 16 = 42.2Now, we need to find 'n' Experimental molecular weight = 165 (average of the given range of 160–170)We can find n byn= (experimental molecular weight) / (empirical formula mass) = 165/42.2 = 3.9 Hence, ‘n’ is approximately 4 Now, 4C2* 4H2* O4 = C8H8O4 Hence, option C is the correct option.

Question 14

The atoms of hemoglobin are heavier than H-atoms:

Accepted Answer

B. 68,000 times — Correct option is B. Hemoglobin is the oxygen carrying molecule in blood. It consists of an iron-containing heme together with 4 globin subunits to form the overall hemoglobin molecule, with molecular weight of about 64.500. Hemoglobin is 68,000 times heavier than hydrogen.

Question 15

If the number of molecules of different gases are the same at S.T.P., the occupied volume will be _:

Accepted Answer

B. Same — At standard temperature and pressure, 1 mole of gas has a volume of 22.4 dm3. This means that every gas occupies the same volume provided that the number of moles is the same.

Question 16

Actual yield is always less than theoretical yield due to:

Accepted Answer

D. All of these — Option D is correct since all three of these reasons are responsible for the actual yield being less than the theoretical yield. Option A is incorrect since operational losses alone are not responsible for the loss in actual yield.Option B is incorrect since reaction being reversible alone is not responsible for the loss in actual yield.Option C is incorrect since side reactions alone are not responsible for the loss in actual yield.

Question 17

It is the fact that 22.414 cm3 of any gas has a different masses but the same number of

Accepted Answer

B. Particles — Option A Different type of gases are madeup of different number of atoms such as argon is made uo of single atom or carbon-dioxide is made up of 3 atoms so A Cannot be the answer Option B 22.414 dm3 of each gas has a different mass however the very same number of particles. The reason is that the masses and the sizes of the monoatomic or diatomic gases don't affect the volumes, so it's correct Option C Not all gases are molecules Option D It is not correct

Question 18

X-ray work in 20th century shows that diameter of the atoms are of the order:

Accepted Answer

B. 0.2nm — X-ray work in the 20th century showed that the diameter of atoms are of order of 0.2 nm.

Question 19

The bond formed when two or more atoms complete their electron shells by sharing electrons is called _:

Accepted Answer

C. Covalent bond — Covalent bonding has a characteristic feature which involves the sharing of electrons between two non-metal atoms.

Question 20

In stoichiometry, we follow law of _ while doing calculations:

Accepted Answer

C. Both A and B — In stoichiometry , the law of conservation of mass and law of definite proportions are obeyed.

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