1. Calculate the number of molecules in 4.5 moles of CO2. The number of molecules in one mole of any substance is given by Avogadro's number, which is $6.022 \times 10^{23}$ molecules/mole. Number of molecules in 4.5 moles = $4.5 \times 6.022 \times 10^{23} = 2.71 \times 10^{24}$ molecules. 2. What is meant by oxidation state? Oxidation state (or oxidation number) is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Two rules for assigning oxidation states: - The oxidation state of an element in its free (uncombined) form is zero. - The oxidation state of oxygen in most compounds is -2. 3. Write the electronic configuration of $^{31}P$. Atomic number of phosphorus (P) is 15. Electronic configuration: $1s^2 2s^2 2p^6 3s^2 3p^3$. Group and Period: - Group: 15 (VA group) - Period: 3 4. Differentiate between crystalline and amorphous solids. - Crystalline solids have a well-ordered, repeating arrangement of atoms. - Amorphous solids lack a long-range order or repeating pattern. 5. Why is an atom always electrically neutral? Because the number of protons (positive charge) equals the number of electrons (negative charge), their charges cancel out, making the atom neutral. 6. What is the composition of aqua regia? Write its importance. Composition: A mixture of concentrated nitric acid (HNO3) and hydrochloric acid (HCl) in a 1:3 volume ratio. Importance: It can dissolve noble metals like gold and platinum which are resistant to single acids. 7. Noble gases show very low reactivity. Explain three properties of their inertness. - They have a complete valence shell, making them stable. - High ionization energies prevent them from losing electrons. - Low electron affinity means they do not easily gain electrons. 8. Differentiate between electronegativity and electron affinity with example. - Electronegativity is the tendency of an atom to attract shared electrons in a chemical bond (e.g., Fluorine has high electronegativity). - Electron affinity is the energy change when an atom gains an electron (e.g., Chlorine has high electron affinity). 9. Briefly explain electroplating of chromium with reactions. Electroplating involves depositing chromium metal on an object using electric current. At cathode: $Cr^{3+} + 3e^- \rightarrow Cr$ (deposition of chromium) At anode: Usually inert anode, no reaction or oxidation of anions. 10. Briefly explain the importance of silver, gold and platinum. - Silver: Used in jewelry, electronics, and photography due to high conductivity and reflectivity. - Gold: Used in jewelry, electronics, and as a standard for monetary systems due to its non-reactivity. - Platinum: Used as a catalyst in chemical reactions and in jewelry due to its resistance to corrosion. 11. State and explain periodic law. Periodic law states that the properties of elements are a periodic function of their atomic numbers. This means elements with similar properties recur at regular intervals when arranged by increasing atomic number.