Solved Questions on Atoms and Molecules
1. When 5 g of calcium carbonate (CaCO3) is heated, it decomposes to form calcium oxide (CaO) and carbon dioxide (CO2). If the residue left after heating weighs 3.4 g, what is the mass of carbon dioxide released?
a) 1.8 g
b) 2.4 g
c) 1.6 g
d) 3.2 g
Answer: c) To solve this question, we can use the concept of conservation of mass. The total mass of the reactants should be equal to the total mass of the products. In this case, the reactant is calcium carbonate (CaCO3), and the products are calcium oxide (CaO) and carbon dioxide (CO2).
Given information:
Initial mass of calcium carbonate (CaCO3) = 5 g
Mass of the residue left after heating = 3.4 g
Let's denote the mass of carbon dioxide released as "m."
According to the conservation of mass, the mass of the reactant (CaCO3) = mass of the products (CaO + CO2):
5 g = (mass of CaO) + m (mass of CO2)
We are given that the mass of the residue (CaO) is 3.4 g. Therefore, the mass of carbon dioxide released (m) can be calculated as:
m = 5 g - 3.4 g
m = 1.6 g
2. The molar mass of element X is 64 g/mol and the molar mass of element Y is 32 g/mol. If the molar mass of the compound they form is 128 g/mol, what is the molecular formula of the compound?
a) XY
b) X2Y
c) XY2
d) X3Y2
Answer: c) Molar mass of element X = 64 g/mol
Molar mass of element Y = 32 g/mol
The molar mass of the compound = 128 g/mol
To find the molecular formula, we need to compare the molar masses of the elements and the compound to see how many atoms of each element are present in the compound.
Let's analyse the options:
a) XY: The molar mass would be 64 g/mol + 32 g/mol = 96 g/mol
b) X2Y: The molar mass would be (2 × 64 g/mol) + 32 g/mol = 160 g/mol
c) XY2: The molar mass would be 64 g/mol + (2 × 32 g/mol) = 128 g/mol (matches the compound's molar mass)
d) X3Y2: The molar mass would be (3 × 64 g/mol) + (2 × 32 g/mol) = 256 g/mol
Among the options, option c) XY2 is the molecular formula that matches the given compound's molar mass (128 g/mol).
Therefore, the correct molecular formula of the compound is XY2.
3. In a chemical reaction, if 4 moles of hydrogen react with 8 moles of oxygen, how many moles of water will be produced?
a) 2 mole
b) 4 moles
c) 6 moles
d) 8 moles
Answer: b) Given the balanced chemical equation:
2H2 + O2 → 2H2O
We see that for every 2 moles of hydrogen (H2), we need 1 mole of oxygen (O2) to form 2 moles of water (H2O). This means that the balanced equation requires an equal number of moles of hydrogen and oxygen to react fully.
If we have 4 moles of hydrogen (H2) and 4 moles of oxygen (O2), we can see that we have twice the required amount of oxygen. This makes oxygen the excess reagent in the reaction, as not all of it will be used up.
In this case, the balanced reaction will only be able to utilize the 2 moles of oxygen with 4 moles of hydrogen to produce 4 moles of H2O.
4. The molar mass of nitrogen gas (N2) is 28 g/mol. How many grams of nitrogen gas are there in 2 moles of N2?
a) 56 g
b) 14 g
c) 28 g
d) 112 g
Answer: a) Molar mass of nitrogen gas (N2) = 28 g/mol
Number of moles of N2 = 2 moles
To find the mass of nitrogen gas (N2) in 2 moles, we can use the formula:
Number of Moles = Mass / Molar Mass
Mass = Molar mass × Number of moles
Plugging in the values:
Mass = 28 g/mol × 2 moles
Mass = 56 g
5. In the following question, you will find an assertion and a reason. Select the appropriate option that applies.
Assertion: The Avogadro number (6.022 x 1023) is used to convert between the number of molecules and the number of moles of a substance.
Reason: Avogadro's number represents the number of particles (atoms, molecules, ions) in one mole of a substance, allowing for easy conversions between these quantities.
a) Both the assertion and reason are correct, and the reason explains the assertion.
b) Both the assertion and reason are correct, but the reason does not explain the assertion.
c) The assertion is correct, but the reason is incorrect.
d) The assertion is incorrect, but the reason is correct.
Answer: a) Both the assertion and reason provided in the question are correct. The assertion states that the Avogadro number is used to convert between the number of molecules and the number of moles of a substance, which is true. The reason provided further explains that Avogadro's number represents the number of particles (atoms, molecules, ions) in one mole of a substance. This accurate representation of Avogadro's number enables easy conversions between these quantities. Since both the assertion and reason are correct, and the reason logically explains the assertion, option "a" is the correct choice.
FAQs
1. What are atoms and molecules?
Atoms are the basic building blocks of matter, consisting of a nucleus containing protons and neutrons, surrounded by electrons. Molecules are groups of atoms bonded together, forming distinct chemical compounds.
2. What are the Different types of atoms?
Atoms are categorized into different groups based on the number of protons in their nucleus, also known as the atomic number. The periodic table classifies atoms into elements, each represented by a distinct chemical symbol.
3. What are some examples of common molecules?
Common molecules include water (H2O), carbon dioxide (CO2), oxygen gas (O2), and glucose (C6H12O6). Each molecule has a unique structure and composition that determines its qualities and functions.
4. How Do Atoms Combine to Form Molecules?
Atoms create molecules via chemical bonds, such as covalent bonds, in which atoms share electrons, and ionic bonds, in which atoms transmit electrons to one another. These bonds lock the atoms together in a stable configuration.
5. What is meant by the term "chemical formula"?
The chemical formula of a substance refers to the symbolic representation of its composition. The chemical formula of a compound tells us the amount and kind of atoms in the distinct elements that make up the complex.
For example, the chemical formula CO2 of carbon dioxide indicates that one carbon atom and two oxygen atoms are chemically bound together to produce a single molecule of the substance carbon dioxide.
