7. A few drops of bromothymol blue indicator are placed into 250 mL of 0.20 mol/L HCl(aq). A sample of 1.8 g of NaOH(s) is then dissolved into this solution. The final colour of the solution is predicted to be
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What is the pH of the HCl solution? THen, dissolve the 1.8 g NaOH. Which is in excess? With that excess, what the the resultant pH?
The pH of the HCl solution is approximately 0.20. After dissolving the 1.8 g of NaOH, the solution will be in excess of the HCl, resulting in a pH of approximately 11. The final colour of the solution will be blue.
To determine the final color of the solution, we need to understand the reactions happening.
First, let's find the pH of the HCl solution. The concentration of HCl is given as 0.20 mol/L. Since HCl is a strong acid, it completely dissociates in water, so the concentration of H+ ions is also 0.20 mol/L. The pH of a strong acid is given by the formula: pH = -log[H+]. Thus, in this case, the pH is -log(0.20) = 0.70.
Next, we dissolve 1.8 g of NaOH in the HCl solution. NaOH is a strong base, so it also completely dissociates in water. The molar mass of NaOH is 22.99 + 16.00 + 1.01 = 40.00 g/mol. Therefore, the number of moles of NaOH can be calculated using the equation: moles = mass / molar mass = 1.8 g / 40.00 g/mol = 0.045 mol.
Since NaOH is a strong base and HCl is a strong acid, they will undergo a neutralization reaction according to the equation: NaOH + HCl → NaCl + H2O. The balanced equation indicates a 1:1 ratio between NaOH and HCl, meaning both react fully with each other.
Given that the concentration of HCl is 0.20 mol/L and the number of moles of NaOH is 0.045 mol, we can determine which chemical is in excess by comparing their stoichiometry. Since the ratio is 1:1, the HCl will be completely consumed first, leaving some NaOH remaining.
To calculate the amount of excess NaOH, we need to subtract the amount reacted with HCl from the total amount of NaOH initially dissolved. Initial moles of NaOH = 0.045 mol - 0.045 mol (reacted with HCl) = 0 mol (excess).
Since NaOH is a strong base, it will undergo hydrolysis in water, causing an increase in OH- concentration. This will result in an increase in the pH of the solution. The more NaOH present, the higher the pH will be. However, in this case, we have 0 moles of excess NaOH, so there will be no significant change in pH due to the dissolved NaOH.
Therefore, the final color of the solution is determined by the pH of the HCl solution, which was found to be 0.70.
To determine the final color of the solution, we first need to find the pH of the HCl solution and then consider the dissolved NaOH.
1. Determine the pH of the HCl solution:
HCl is a strong acid, and when it's dissolved in water, it completely dissociates into H+ and Cl- ions. Since the concentration of HCl is given as 0.20 mol/L, the concentration of H+ ions in the solution is also 0.20 mol/L.
pH is a logarithmic scale used to measure the acidity or basicity of a solution. It can be calculated using the formula: pH = -log10[H+].
In this case, the pH of the HCl solution is given by pH = -log10(0.20).
2. Dissolve the 1.8 g of NaOH:
NaOH is a strong base, and when it's dissolved in water, it completely dissociates into Na+ and OH- ions. The molar mass of NaOH is 40.00 g/mol, so we can convert the mass of NaOH to moles by dividing it by the molar mass.
Moles of NaOH = mass of NaOH / molar mass of NaOH.
3. Determine which reactant is in excess:
To determine which reactant is in excess, we compare the number of moles of H+ ions and OH- ions. The balanced equation for the reaction between HCl and NaOH is:
HCl + NaOH → NaCl + H2O
The ratio between H+ and OH- ions is 1:1 in a neutralization reaction. If there are any remaining H+ or OH- ions, the solution will be acidic or basic, respectively.
4. Calculate the resultant pH:
If NaOH is in excess, all the H+ ions will react with OH- ions to form water. In this case, the resultant solution will be neutral with a pH of 7.
If HCl is in excess, the remaining H+ ions will make the solution more acidic, resulting in a lower pH.
Finally, based on the calculated pH and whether HCl or NaOH is in excess, we can determine the predicted color of the solution using bromothymol blue indicator's color chart or pH range.