CHEM-CLICK SK015 - Green Zone Chapter 1

CHAPTER 1

Question 1

(a) An ion Y3+ has 18 electrons and 24 neutrons.

i. Determine the proton number and nucleon number of element Y.

ii. Write the isotopic notation to represent the element.

(b) A student would like to prepare 250 mL of 0.25 mol L-1 lead (II) nitrate, Pb (NO3)2 solution.

i. Calculate the mass of Pb (NO3)2 required to prepare the solution.

ii. Based on the information in b(i), give TWO (2) main steps that should be carried out to prepare the solution .

(c) For the following redox equation (not balanced), choose a pair of species that undergo reduction. Explain.

Br-(aq) + MnO4-(aq) → BrO3-(aq) + MnO2(s)

Question 2

(a) Thallium has two naturally occurring isotopes, 203Tl and 205Tl, with percentage abundance of 29.5% and 70.5%, respectively. Calculate the average atomic mass of thallium.

(b) 18.3 g sample of a hydrated compound contained 4.0 g of calcium, 7.1 g of chlorine and 7.2 g of water. Calculate its empirical formula.

(c) A sample of 0.52g of NaNO2 powder is weighed and dissolved in distilled water and made up to the mark in 100ml volumetric flask. 25ml of the solution is titrated with K2Cr2O7 solution which oxidises NO2- to NO3- while Cr2O72- ions are reduced to Cr3+ ions. 30ml of K2Cr2O7 solution is required to reach the equivalence point.

i. Write the balanced equation for the titration.

ii. Calculate the molarity of the NaNO2 solution prepared.

iii. Determine the molarity of K2Cr2O7 solution used in the titration.

iv. What is the mass of water produced in the reaction.

v. If the percentage yield of water produced is 80%, calculate the actual mass of water produced during the experiment.

(a) Compound A contains carbon, hydrogen and nitrogen. Combustion of 0.250 g of A produces 0.344 g of water, H2O, and 0.558 g of carbon dioxide, CO2. Determine the molecular formula of A if the molar mass of A is 59 g mol-1.

Question 3

(b) A solution was prepared by dissolving 25 g of anhydrous oxalic acid, H2C2O4, in distilled water in a 250 mL volumetric flask.

i. Calculate the molarity of the resulting solution.

ii. The resulting solution has the density of 1.90 gcm-3. Calculate the percent by mass of the solution.

Question 4

(a) A certain amount NaOH is dissolved in 98 g of water at 250C. The final volume of NaOH solution is 100 mL. Given the density of the solution is 1.33 g mL-1 at 250C, calculate the number of moles of NaOH used.

(b) In an experiment, 0.355 g of aluminium, Al reacts with 20.00 mL of 4.00 M hydrochloric, HCl according to the following equation:

2 Al (s) + 6 HCl (aq) → 2 AlCl3 (aq) + 3H2 (g)

i. Determine the limiting reactant.

ii. If the percentage yield of hydrogen is 85%, calculate the actual mass of hydrogen obtained