Data ------ Beaker: 10.0 mL of 1.018 M HCl, + 100.0 mL distwilled water Mass ini
ID: 1037610 • Letter: D
Question
Data
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Beaker: 10.0 mL of 1.018 M HCl, + 100.0 mL distwilled water
Mass initial: .295 g
Mass final: .215 g
Question
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1) Why does the pH of the acid solution initially increase very slowly when metal is first added to the acid solution, but gradually increase at a faster rate as the reaction proceeds to completion?
2) How do you determine the molar mass of the unknown metal?
Experiment was basically a beaker full of HCl, slowly add the unknown metal to it, watch the pH rise with the measurenet system and then when it hit a certain point we just stopped. We did not take down data point of pH. The aim of the procedure is to identify the unknown metal by determining its molar mass, basically.
Explanation / Answer
1) Let the metal be denoted as M and the valence of the metal is n; therefore, the balanced chemical equation for the reaction is
M (s) + n HCl (aq) --------> MCln (aq) + n/2 H2 (g)
As per the stoichiometric equation,
1 mole M = n moles HCl = n/2 moles HCl.
The net ionic equation for the reaction is
M (s) + n H+ (aq) --------> Mn+ (aq) + n/2 H2 (aq)
Initially the beaker contains only HCl (which ionizes completely to produce H+) and the metal is dropped into the beaker. The metal reacts with H+ as shown by the ionic equation to produce H2. However, since the acid is present in large proportion as compared to the metal (at the beginning of the reaction), the concentration of H+ is not significantly lowered. The pH of the solution is defined as pH = -log [H+]. Since [H+] remains fairly constant at the start of the reaction, hence, the pH increases slowly at the start of the reaction. As the reaction proceeds, H+ is converted to H2 and hence, [H+] decreases; therefore, as the reaction progresses, the pH increases sharply.
2) We see from the stoichiometric equation that
1 mole M = n/2 moles of H2.
We can treat the H2 as an ideal gas and find out the mole(s) of H2 produced by noting down the pressure of dry H2 gas and the volume of the gas (can be recorded from the volume of water displaced). By noting the temperature of the gas, we can calculate the moles of H2 produced and hence, the moles of the metal M. The mass of the metal M taken is known. The molar mass of the metal is given as
Molar mass of metal M = (mass of metal M)/(moles of metal M)
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