Some Basic Concepts in Chemistry

21 solved questions 1 test

Ask a question

Take chapter test Next chapter →

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... Atomic and molecular masses, mole concept, molar mass 1 answer
Dalton's Atomic theory could not explain which of the following? Answer: 4. Law of gaseous volume Key idea: Dalton's theory was about atoms combining in fixed whole-number ratios, with no concept of gas vo... Matter and its nature, Dalton's atomic theory 1 answer
A compound X contains 32% of A, 20% of B and remaining percentage of C. Then, the empirical formula of X is :(Given atomic masses of A = 64; B = 40; C = 32 u) Answer: 3. ABC₃ Key idea: empirical formula from percentage: divide by atomic mass, then by the smallest value. C% = 100 - 32 - 20 = 48. Ele... Percentage composition, empirical and molecular formulae 1 answer
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... Atomic and molecular masses, mole concept, molar mass 1 answer
1 gram of sodium hydroxide was treated with 25 mL of 0.75 M HCl solution, the mass of sodium hydroxide left unreacted is equal to Answer: 3. 250 mg Key idea: find the limiting reagent for NaOH+HCl→ NaCl+H₂O . NaOH: 1/40=0.025 mol = 25 mmol. HCl: 25×0.75=18.75 mmol. Left... Chemical equations and stoichiometry 1 answer
Which amongst the following options is correct graphical representation of Boyle's Law?(1) P vs T, straight lines through origin labelled V_1, V_2, V_3 with V_1<V_2<V_3; (2) P vs V, hyperbolas labelled T_1, T_2, T_3 with T_3>T_2>T_1; (3) P vs 1/V, st… Answer: 3. P vs 1/V lines, T ₃ >T ₂ >T ₁ (graph) Key idea: Boyle's law: at constant T, P ∝ 1/V , so P plotted against 1/V is a straight line... Chemical equations and stoichiometry 1 answer
The right option for the mass of CO_2 produced by heating 20 g of 20% pure limestone is (Atomic mass of Ca = 40)CaCO_3 1200\ K CaO + CO_2 Answer: 3. 1.76 g Key idea: Only the pure CaCO₃ decomposes; find its moles and use 1:1 mole ratio with CO₂ . Pure CaCO₃ = 20 × 20/100 = 4 g.... Chemical equations and stoichiometry 1 answer
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... Atomic and molecular masses, mole concept, molar mass 1 answer
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... Atomic and molecular masses, mole concept, molar mass 1 answer
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... Atomic and molecular masses, mole concept, molar mass 1 answer
What mass of 95% pure CaCO_3 will be required to neutralise 50 mL of 0.5 M HCl solution according to the following reaction ?CaCO_3(s) + 2HCl_(aq) → CaCl_2(aq) + CO_2(g) + 2H_2O_(l)[Calculate upto second place of decimal point] Answer: 3. 1.32 g Key idea: find moles of HCl, use the 1 : 2 stoichiometry, then correct for purity. Moles of HCl =0.5×0.050=0.025 mol. CaCO... Chemical equations and stoichiometry 1 answer
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... Atomic and molecular masses, mole concept, molar mass 1 answer
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... Atomic and molecular masses, mole concept, molar mass 1 answer
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).... Atomic and molecular masses, mole concept, molar mass 1 answer
The number of protons, neutrons and electrons in ^175_71Lu, respectively, are : Answer: 1. 71, 104 and 71 For ^A_ZX : protons =Z=71 ; neutrons =A-Z=175-71=104 ; electrons (neutral atom) =71 . Answer: 71, 104 and 71, opti... Matter and its nature, Dalton's atomic theory 1 answer
The number of moles of hydrogen molecules required to produce 20 moles of ammonia through Haber's process is : Answer: 3. 30 N₂ + 3H₂ → 2NH₃ : 3 mol H₂ give 2 mol NH₃ . Moles of H₂ = 20×3/2 = 30 . Option (3). Chemical equations and stoichiometry 1 answer
A gas at 350 K and 15 bar has molar volume 20 percent smaller than that for an ideal gas under the same conditions. The correct option about the gas and its compressibility factor (Z) is : Answer: 3. Z < 1 and attractive forces are dominant Compressibility factor Z = V_real/V_ideal . The molar volume is 20% smaller, so V_real =... Matter and its nature, Dalton's atomic theory 1 answer
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... Atomic and molecular masses, mole concept, molar mass 1 answer
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... Atomic and molecular masses, mole concept, molar mass 1 answer
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... Atomic and molecular masses, mole concept, molar mass 1 answer
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... Atomic and molecular masses, mole concept, molar mass 1 answer