Part 15 Analytical Chemistry Flashcards
Suggest a test for H2 (g).
It burns with pop sound.
Suggest a test for O2 (g).
It relights glowing splint.
Suggest a chemical test for CO2 (g).
It turns limewater milky. Then the milky solution becomes colourless.
Suggest a chemical test for Cl2 (g).
It turns moist blue litmus paper red then bleaches it very quickly.
Suggest a chemical test for NH3 (g)
It turns moist litmus paper blue // bring the gas close to an unstoppered bottle of conc. HCl. Dense white fumes formed.
Suggest a chemical test for HCl (g).
Bring the gas close to an unstoppered bottle of conc. NH3(aq). Dense white fumes formed.
Suggest a chemical test for SO2 (g).
It turns limewater milky // it turns a piece of filter paper soaked with acidified K2Cr2O7 solution from orange to green // it turns brown bromine water colourless // it decolourises petals temporarily // pass the gas to (Cr2O7)2- / H+ (or (MnO4)- / H+. The solution turns from orange to green.
Suggest a test for H2O (g).
It turns dry cobalt(II) chloride paper from blue to pink. // it turns white anhydrous copper(II) sulphate powder blue.
Suggest a test for Ca2+ ions.
Flame test, It burns with brick-red flame.
Suggest a test for Cu2+ ions.
It burns with bluish-green flame.
Suggest a test for K+ ions.
It burns with lilac flames.
Suggest a test for Na+ ions.
It burns with golden yellow flame.
Suggest a chemical test for Cl- ions.
Add acidified silver nitrate solution.
White precipitate formed.
Suggest a chemical test for Br- ions.
Add acidified silver nitrate solution.
Pale yellow precipitate formed.
Suggest a chemical test for OCl- ions in aqueous solution.
Add a few drops of HCl (aq).
Pale yellowish-green gas evolved which turns moist blue litmus paper red then bleaches it.
Suggest a chemical test for (SO3)2- ions
Add Br2(aq), the solutions changes from brown to colourless. // Add (Cr2O7)2- / H+ , the solution changes from orange to green. // Add MnO4- / H+ , the solution changes from purple colourless.
suggest a chemical test for alkene. (C=C)
add Br(in organic solvent). The bromine solution changes from orange to colourless rapidly. // add (Cr2O7)2- / H+. The solution changes from purple to colourless.
suggest a chemical test for hydroxyl group. (–OH)
Warm with (Cr2O7)2- / H+. The solution changes from orange to green for 1° and 2° alcohol.
Suggest a chemical test for alkene.( >C=C< )
Add Br(in organic solvent). The bromine solution changes from orange to colourless rapidly. // Add MnO4- / H+. The solution changes from purple to colourless.
Suggest a chemical test for hydroxyl group.( –OH )
Warm with (Cr2O7)2- / H+. The solution changes from orange to green for 1° and 2° alcohol. // Heat with a carboxylic acid in the presence of concentrated sulphuric acid. An ester with a characteristic pleasant fruity smell forms.
Suggest a chemical test for aldehydes and ketone.( >C=O )
Add 2-4-dinitrophenylhydrazine solutions. Red, yellow or orange precipitate formed
suggest a chemical test for aldehydes.( –CHO )
Warm with Tollens’ reagent. A silver mirror forms on the inner wall of the test tube. // Add acidified K2Cr2O7 / H+. The solution turns from orange to green.
Suggest a chemical test for Carbonyl group.( –COOH )
Heat with an alcohol in the presence of concentrated sulphuric acid. An ester with a characteristic pleasant fruity smell forms.
What is meant by the term “Rf value” ?
Rf value of a substance is the ratio between the migration distance of the substance and the migration distance of the solvent front during chromatography.
Suggest one method that can be used to make the two spots in the chromatographic plate become visible.
Put the chromatographic plate into a jar that is saturated with iodine vapur. The spots will appear brown. // Irradiate the plate with UV. The stationary phase is flourescent while the two spots are not.
Explain briefly why chromatography can be used to seperate a mixture
Different substance have different adsorptivity to the stationary phase and different solubility in mobile phase.
Suggest how you can identify that the chemical you obtained is pure.
Run a TLC / paper chromatography for the sample; if only one spot is detected on the chromatogram, the chemcial obtained is pure // (if the m.p / b.p. is given)Measure the m.p. / b.p. of the chemical obtained if the m.p. / b.p. is (…) it may be pure. //
Explain why filtration can be used to remove mud particals from muddy water, but cannot be used to remove sodium chloride from sea water.
Sea water is solution while muddy water is suspension. The size of ions in water is much smaller than that of the mud particals in muddy water. Ions in sea water can pass through pores in filter device while mud particals cannot.
Suggest one property of solvents rendering them to be used in the liquid-liquid extraction.
The solvents are immiscible with water // The solvents do not react with the organic compounds // The solvents can dissolve more oragnic compounds than water can // The solvents have low boiling points / can evaporate easily
Why are standard iodine solutions not prepared directly from solid iodine ?
I2(s) is very volatile. It is difficult to weigh accurately a sample of I2 // I2 reaxts readily with reducing agents in the envirnent. I2(s) in laboratory usually contains impurities
Why is SO2 is added to white wine ?
SO2 acts as an antioxidant in wine. It helps prevent the oxidation of ethanol to ethanoic acid by the actions of oxygen and bateria in air
State one condition that a reaction must fulfill in order that it can be used in volumetric analysis.
A significant observable change can be easily be detected with the complete consumption of the reagent in the conical flask, i.e. availability of suitable indicator // The reaction must be significantly complete // The rate of reaction must be fast enough to be practical
What is meant by the term “primary standard” ?
A substance which can be used to prepare standard solution by dissolving a known mass of the substance in a solvent and making up the solution to a known volume
Explain why NaOH / KOH cannot be used as primary standard.
NaOH / KOH can absorb water and react with CO2 in air. Mass of NaOH cannot be accurately measured
