Atomic and molecular masses, mole concept, molar mass
- Among the following, choose the ones with equal number of atoms.A. 212 g of Na_2CO_3 (s) [molar mass = 106 g]B. 248 g of Na_2O (s) [molar mass = 62 g]C. 240 g of NaOH (s) [molar mass = 40 g]D. 12 g of H_2 (g) [molar mass = 2 g]E. 220 g of CO_2(g) [mo… Answer: 2. A, B, and D only Key idea: number of atoms = moles × atoms per formula unit × N_A . A: 212 g = 2 mol × 6 = 12 mol atoms. B: 248 g... 1
- The highest number of helium atoms is in Answer: 2. 4 mol of helium Key idea: convert each to number of atoms (He is monoatomic). 2.271098 L at STP (22.7 L/mol): 0.1 mol. 4 mol: 4 m... 1
- A 10.0 L flask contains 64 g of oxygen at 27°C. (Assume O_2 gas is behaving ideally). The pressure inside the flask in bar is(Given R = 0.0831 L bar K^-1 mol^-1) Answer: 1. 4.9 Key idea: use PV=nRT . n=64/32=2 mol, T=300 K. P=nRT/V=2×0.0831×300/10=4.986≈4.9 bar (nearest option is 4.9; 4.986 bar is 4.9... 1
- Copper crystallises in fcc unit cell with cell edge length of 3.608 × 10^-8 cm. The density of copper is 8.92 g cm^-3. Calculate the atomic mass of copper. Answer: 2. 63.1 u Key idea: density ρ=ZM/a³N_A , so M=ρ a³N_A/Z . fcc has Z = 4. a³=(3.608×10⁻⁸)³=4.697×10⁻²³\ cm³ . M=dfrac8.92×4.697×10⁻²³... 1
- Which one is not correct mathematical equation for Dalton's Law of partial pressure ? Here p = total pressure of gaseous mixture Answer: 1. p_i = chi_i p_i^° , where chi_i = mole fraction of i^th gas in gaseous mixture, p_i^° = pressure of i^th gas in pure state Key id... 1
- Tritium, a radioactive isotope of hydrogen, emits which of the following particles ? Answer: 1. Beta ( β^- ) Key idea: tritium ( ³₁H ) is a beta emitter. ³₁H→³₂He+β^-+barν : a neutron changes into a proton, emitting a β^- par... 1
- Choose the correct option for the total pressure (in atm.) in a mixture of 4 g O_2 and 2 g H_2 confined in a total volume of one litre at 0^° C is :[Given R=0.082 L atm mol^-1K^-1, T=273 K] Answer: 3. 25.18 Key idea: PV=nRT with total moles of gas. n_(O_2)=4/32=0.125 ; n_(H_2)=2/2=1 ; total = 1.125 mol. P=nRT/V=1.125×0.082×273/1... 1
- Which one of the followings has maximum number of atoms ? Answer: 4. 1 g of Li(s) [Atomic mass of Li = 7] Number of atoms is proportional to moles of atoms = mass / atomic mass (all masses are 1 g).... 1
- In which case is the number of molecules of water maximum ? Answer: 1. 18 mL of water Compare the number of moles; more moles means more molecules. 18 mL water = 18 g (density 1 g/mL) = 1 mol. 0.00224... 1
- Suppose the elements X and Y combine to form two compounds XY_2 and X_3Y_2. When 0.1 mole of XY_2 weighs 10 g and 0.05 mole of X_3Y_2 weighs 9 g, the atomic weights of X and Y are Answer: 4. 40, 30 Key idea: molar mass = mass / moles, then form two equations in the atomic masses X and Y. XY₂ : M=10/0.1=100⇒ X+2Y=100 X₃... 1
- Lithium has a bcc structure. Its density is 530\ kg\ m^-3 and its atomic mass is 6.94\ g\ mol^-1. Calculate the edge length of a unit cell of Lithium metal. (N_A = 6.02×10^23\ mol^-1) Answer: 2. 352 pm Key idea: ρ=Z M/N_A a³ , with Z = 2 for bcc. ρ=530\ mathrmkg m⁻³=0.530\ mathrmg cm⁻³ . a³=frac2×6.946.02×10²³×0.530=4.35×1... 1
- Equal moles of hydrogen and oxygen gases are placed in a container with a pin-hole through which both can escape. What fraction of the oxygen escapes in the time required for one-half of the hydrogen to escape ? Answer: 1. 1/8 Key idea: Graham's law, rate of effusion ∝ 1/√(M) . fracr_(H_2)r_(O_2)=√(32/2)=4 Both gases start with equal moles and leave... 1