Given a sample of 3.0 M hydrochloric acid, phenolphthalein, and some common laboratory equipment, de — Qualitative Analysis and Gas Stoichiometry Chemistry Question
Problem Context
Given a sample of 3.0 M hydrochloric acid, phenolphthalein, and some common laboratory equipment, devise an experiment using both qualitative and quantitative evidence to determine the provided unknown metal given these possible choices: Ag, Al, Ca, or Cr.
Give a brief description of your experimental plan.
Model Answer
Students were to provide both qualitative and quantitative evidence to determine the unknown metal. The metal provided was calcium. The results to this experiment should have included both evidence form data obtained and exclusive information about what was not observed from students’ previous chemical knowledge. Conclusions come from knowledge about each metals’ reactivity to both water and HCl, with phenolphthalein, a possible titration, and gas generation. Students might also have explored reactivity of the metal with NaOH from Problem #1 (this is allowed, though not necessary to successfully complete this problem).
Excellent work: Student combined HCl with the unknown metal (Ca) to obtain hydrogen gas in the wellplate, clearly showing evidence of gas production and exothermic reaction. The student then performed this reaction with a measured amount of Ca and excess HCl using the Luer-Lok syringe to quantify the hydrogen gas produced (since room temperature and pressure were not given, student had to make some assumptions about the Kelvin temperature and room pressure, perhaps estimating 298K and 1 atm) to determine the expected volume of hydrogen and compare it to a theoretical volume produced from Ca + HCl → CaCl2 + H2. Noting the color change when phenolphthalein is added to the metal reacted to either water or HCl. It is possible that a student might have thought to combine mustard (from Prob.#1) with the metal from this experiment. If so, mustard on the surface of Ca produces over time a crusty white solid, Ca(C2H3O2)2 (there is no evidence of reaction with mustard on the surface of Cr and with pure Ag, no visible reaction).
Concluding what DIDN’T occur:
If Cr + HCl → greenish color indicating CrCl3 (or green color with many chromium salts)
If Ag + HCl → no reaction
If Al + HCl → no visible reaction due to aluminum oxide layer (though student might have attempted to dissolve the metal with the NaOH from Exp. #1, if Al, would dissolve; Ca + NaOH → gives Ca(OH)2, a noticeable milky white precipitate, with phenolphthalein produces a pink color.
a) Sample titration experiment conclusions:
Reacting a 0.10 g metal turning with water completely, adding phenolphthalein, then titrating with the 3M HCl to obtain a 2 : 1 ratio of OH- : H+ in solution, confirms that OH- must be present in the metal hydroxide form, M(OH)2.
b) Sample data for quantifying hydrogen gas generated using the Luer-Lok® syringe:
One metal turning, approx. 0.07 g Ca in excess 3M HCl
Begin at 12 mL mark on syringe
End at 40 mL mark on syringe
40 – 12 = 28 mL hydrogen gas generated, strongly exothermic reaction.
Assume room temp. 25oC (298K) and 1 atm:
using the ideal gas law, PV = nRT
(1 atm)(0.028L) = n (0.0821atm L/mol K) (293K) ; n = 0.00116 mol H2(g)
if given 0.07g of calcium, Ca + 2HCl → CaCl2(aq) + H2(g) , then 0.0035 g of hydrogen gas is produced, corresponds roughly to number of moles of H2(g) made with these assumed conditions.
Average work: Student reacted metal with HCl and concluded hydrogen gas was present but didn’t quantify the gas produced, or did but incorrectly. Student wrote out possible reactions with the other possibilities but did not do so correctly.
Below average work: Student was unable to conclude that hydrogen gas was produced, did not use either a titration or quantitative method of data collection, or unable to use the phenolphthalein to qualitatively justify the metal.
Record your data and other observations.
Model Answer
(See comments above)
Conclusions and Evidence.
Model Answer
The unknown metal is = __Calcium__________