Note: Respond To All Questions From Part A To C A. When carbon dioxide and water

Question

Note: Respond To All Questions From Part A To C

A. When carbon dioxide and water mix. carbonic acid is produced, pH is a measure of the acidity or basicity(alkalinity) of a material when dissolved in water. It is expressed on a scale from 0 to 14. Phenolphthalein is a chemical indicator which may be used to monitor pH change in a solution. Phenolphthalein will change color at a pH of 8.2. At alkaline pH's, above pH 8.2, phenolphthalein is pink. When the pH is below 8.2, phenolphthalein is clear. Lab instructor has a beaker containing the same phenolphthalein solution as used above. Observe what occurs to this solution as the instructor exhales through the straw into the solution. Describe what happened. B. What major metabolic pathway(s) are occurring in plants: - In the beaker placed in the dark? Explain In the beaker placed in the light? Explain C. The plant in the dark should actually require the most drops of base for titration, even more than the control beaker with no plant. What does this mean in terms of CO usage or production by the plant in the dark? Do plants undergo cellular respiration?

Explanation / Answer

A. pH is the potential hydrogen ion concentration. It indicates the concentration of hydrogen ions [H+]. It tells whether a solution is acidic or alkaline. Acid is a substance that donates H+ ions, whereas base is a substance that accept H+ ions.

Phenolphthalein is an acid/base indicator. It is colorless at an acidic pH (below 8.2) and pink at alkaline pH (above 8.2).

Exhaled air is rich in carbon dioxide (CO2). Hence, when exhaled into the solution, CO2 in the person’s breath reacts with water to form Carbonic acid (H2CO3).

CO2 + H2O H2CO3

This carbonic acid dissociates to form free hydrogen ion and bicarbonate.

H2CO3 HCO3- + H+

This may further dissociate forming carbonate and free hydrogen.

HCO3- CO32- + H+

Hence, the reaction can be summed up as follows:

CO2 + H2O H2CO3 HCO3- + H+ CO32- + 2H+

Since H+ ions are produced, the solution becomes acidic leading to decrease in the pH. Therefore, upon exhalation into the solution, the solution becomes colorless indicating decrease in pH and acidic nature.

B. A plant kept in the dark will not undergo photosynthesis due to the absence of light. But it carries out cellular respiration in the mitochondria. This involves the breaking down of food molecules to obtain energy and store in the form of ATP. They use the product of glycolysis, the pyruvate molecules to release energy along with carbon dioxide and water as the byproducts of the reaction. Hence, when a plant is kept in the dark for a long time excess amount of carbon dioxide production will happen.

But a plant kept in the light will undergo photosynthesis as well as cellular respiration. Photosynthesis make use of carbon dioxide and water to make glucose, oxygen and energy. Cellular respiration make use of these products to obtain energy-storing ATP molecules and carbon dioxide as the by-product.

C. Titration involves gradual addition of a titrant solution to an analyte solution. The titrant has a known concentration while analyte has unknown concentration. In order to determine carbon dioxide levels in a solution, a base like sodium hydroxide is used to titrate. The carbon dioxide will react with sodium hydroxide to form sodium bicarbonate. By using phenolphthalein, we can determine when all the carbon dioxide in water has been consumed.

Plants in the dark produce more carbon dioxide due to cellular respiration. A control beaker will not have carbon dioxide in them. So, beaker containing the plant in the dark will require the most drops of base till the all of carbon dioxide react with the base or has been consumed. When a solution turns pink on addition of phenolphthalein, it shows all the carbon dioxide has been consumed. But if it is still clear, u will need to add more base to it. Hence, the beaker containing the plant in the dark will require more drops of base than the control beaker.

This shows the plant in the dark has undergone cellular respiration and hence, carbon dioxide is present in that particular solution.

Related Questions

Navigate

• Browse (All)
• Browse N
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.