#11)
The information:
An unidentified acid with a concentration of 1.0 mol/L has been given to you to
identify. The acid appears in your data booklet on the Relative Strengths of Acids
and Bases table. The following test results were recorded:
1. Methyl violet is yellow when added to the acid.
2. The acid did not form a precipitate when a solution containing Ag + (aq) was
added to it.
3. The solution turned blue and a gas was formed when a strip of copper was
added to the acid.
The question:
Based on these test results, identify the acid and justify your choice. Your answer
should include equations and/or calculations where appropriate.
My question:
How do I go about doing this? I really have no idea! Please help!
1) the color of the indicator tells you it has a very low pH, indicating a strong acid.
2) No precipate..not HCl, nor H2SO4...silver sulfate is slightly soluble, no way a 1M solution would be soluble.
3) OK, now of the three common strong acids, HCl, H2SO4, and HNO3, which will leave a blue solution with a gas (not hydrogen) being released when copper is in acid?
I agree that it PROBABLY is HNO3.
AgBr, AgCl, and AgI are insoluble so that eliminates HCl, HI, and HBr. Ag2SO4 is only slightly soluble but I rule that out, also, because H2SO4 reacts with Cu but only if the solution is hot (SO2 is given off and blue CuSO4 is formed). AgClO4 is freely soluble and AgClO3 rather soluble but HClO3 and HClO4 are not common acids; therefore, I eliminate those but they could be possibilities because there is no mention of the color of the gas (hydrogen is colorless and odorless). That leaves only the one common strong acid, HNO3. It would have been nice to say that the gas emitted was not colorless or that it was not odorless (that would have positively eliminated HClO4 and HClO3. I hope this helps. Post the balanced equations if you would like for us to check them for you.
To identify the unknown acid, we need to analyze the information provided and make some observations:
1. Methyl violet is yellow when added to the acid:
- Methyl violet is an acid-base indicator that changes color based on the pH of the solution.
- In this case, the fact that it turns yellow indicates that the acid has a very low pH, suggesting it is a strong acid.
2. No precipitate formed when a solution containing Ag+ (aq) was added:
- This eliminates hydrochloric acid (HCl) and sulfuric acid (H2SO4) because they would form insoluble precipitates with silver ions.
- For example, HCl would form silver chloride (AgCl), which is insoluble.
- Based on this, we can rule out these acids.
3. The solution turned blue and gas was formed when a strip of copper was added:
- This observation narrows down the possibilities even further.
- We know the acid is strong because of the methyl violet test. So, we're looking for a common strong acid that reacts with copper.
- Hydrochloric acid is not a candidate because it would produce hydrogen gas (H2) instead of a blue solution.
- Sulfuric acid could react with copper, but it requires heat and wouldn't produce a blue solution with a gas being released.
- This leaves us with nitric acid (HNO3) as the most likely candidate for the unknown acid.
- When copper reacts with nitric acid, it produces copper(II) nitrate (Cu(NO3)2) and nitrogen dioxide gas (NO2):
3Cu(s) + 8HNO3(aq) → 3Cu(NO3)2(aq) + 2NO2(g) + 4H2O(l)
Based on the observations and deductions made, it is highly probable that the unknown acid is HNO3 (nitric acid).