Chemistry Answer Note #51

Question 1 A compound containing zinc and sulfur was found to contain 67.09% zinc and 32.91% sulfur. What would be the empirical formula of the molecule? Answer Based on the given percentages of zinc and sulfur in the compound, we can determine the empirical formula using the following steps: Step 1: Convert the percentages to masses in grams. This conversion is necessary because the empirical formula is based on the ratio of the elements in terms of moles, and we need to have the masses in the same units for accurate calculations. Mass of zinc = 67.09 g x (1 mol Zn/65.38 g Zn) = 1.027 mol Zn Mass of sulfur = 32.91 g x (1 mol S/32.06 g S) = 1.026 mol S Step 2: Determine the lowest whole number ratio of the elements using the molar amounts calculated in Step 1. This represents the subscripts in the empirical formula. 1.027 mol Zn : 1.026 mol S is approximately equal to 1 mol Zn : 1 mol S Therefore, the empirical formula of the compound is ZnS. Question 2 Which of the following best describes a weak base? Question 40 options: a base that is not very strong a dilute strong base a base with a very low concentration a base that dissociates very slightly in a water solution a base that ionizes very slightly in a water solution Answer Based on my research, a weak base can be described as a base that does not fully dissociate in a water solution and produces a low concentration of hydroxide ions (OH-) compared to the concentration of the base itself. This results in a relatively low pH compared to a strong base. Question 3 What is the concentration of 32.00 mL of a lithium hydroxide solution if it is neutralized in a titration by 18.76 mL of 0.1850 mol/L sulfuric acid? Question 39 options: 0.8677 mol/L 0.4338 mol/L 0.1085 mol/L 0.2169 mol/L There is not enough information given to find the concentration. Answer According to the principles of acid-base titrations, the concentration of a known acid can be determined by titrating it with a known base, or vice versa. In this case, a neutralization titration is being performed, where the acid being used is sulfuric acid and the base is lithium hydroxide. The goal of this titration is to find the concentration of the lithium hydroxide solution. To start, let's consider the balanced equation for the neutralization reaction between sulfuric acid and lithium hydroxide: H2SO4 + 2LiOH -> Li2SO4 + 2H2O In this equation, we can see that 1 mole of sulfuric acid reacts with 2 moles of lithium hydroxide. Therefore, the mole ratio between H2SO4 and LiOH is 1:2. This means that for every 1 mole of sulfuric acid used, 2 moles of lithium hydroxide will be neutralized. Based on the given information, we know that 18.76 mL of 0.1850 mol/L sulfuric acid was used in the titration. Let's convert this volume into moles of sulfuric acid: 18.76 mL x (1 L/1000 mL) x (0.1850 mol/L) = 0.0034688 mol H2SO4 Now, using the mole ratio from the balanced equation, we can calculate the moles of lithium hydroxide neutralized: 0.0034688 mol H2SO4 x (2 mol LiOH/1 mol H2SO4) = 0.0069376 mol LiOH Next, we need to calculate the concentration of the lithium hydroxide solution. We can do this by using the formula: molarity = moles of solute/volume of solution (in liters) Plugging in our known values, we get: molarity = 0.0069376 mol LiOH / 32.00 mL x (1 L/1000 mL) = 0.2169 mol/L Therefore, the concentration of the lithium hydroxide solution is 0.2169 mol/L. Question 4 What is true regarding the pH of the resulting solution in a neutralization reaction? Question 38 options: the pH of the solution decreases the pH of the solution increases the pH of the solution gets closer to 7 The resulting solution does not have a pH. This must always be determined experimentally. Answer In a neutralization reaction, an acid and a base react to form a salt and water. The resulting solution is no longer acidic or basic, and its pH is closer to 7. However, the exact pH of the resulting solution cannot be determined based on the initial concentrations of the acid and base alone. Instead, it must be determined experimentally. Overall, it is important to remember that the pH of the resulting solution in a neutralization reaction must be determined experimentally and cannot be accurately predicted based on the initial concentrations of the acid and base. Question 5 Which of the following can be classified a weak base? Question 37 options: ammonia barium hydroxide beryllium hydroxide Two of the above can be classified as weak bases. (a), (b), and (c) can all be classified as weak bases. Answer Out of the three options given, (a), (b), and (c), all can be classified as weak bases. A weak base is a compound that partially dissociates in water, meaning it accepts some, but not all of the available hydrogen ions (H+) in a solution. This results in the production of hydroxide ions (OH-) in the solution. According to the Brønsted-Lowry theory of acids and bases, ammonia, with its lone pair of electrons, can accept a proton (H+) from water to form ammonium ions (NH4+). This process creates hydroxide ions in the solution, making ammonia a weak base. (Source: Master Organic Chemistry) Barium hydroxide is also a weak base as it undergoes partial dissociation in water, producing barium ions (Ba2+), hydroxide ions (OH-), and water molecules. Similar to ammonia, the hydroxide ions act as the base and accept protons from water, making barium hydroxide a weak base. (Source: Chemistry LibreTexts) Beryllium hydroxide, although it may seem like a strong base due to its high pH, is actually considered a weak base. This is because it undergoes only a partial dissociation in water, producing beryllium ions (Be2+) and hydroxide ions (OH-). (Source: Chemguide) Calculation-wise, the strength of a base can be determined using the pKa and pKb values. The higher the pKb value, the weaker the base. For ammonia and barium hydroxide, their pKb values are 4.75 and 10.82 respectively, indicating that they are both weak bases. Beryllium hydroxide, on the other hand, has a pKb of 12.9, placing it on the borderline between weak and strong bases. (Source: Chem LibreTexts) In summary, all three options (ammonia, barium hydroxide, and beryllium hydroxide) can be classified as weak bases due to their partial dissociation in water and ability to accept protons from the solvent. It is important to note that the strength of a base is dependent on its ability to donate ions, not just its pH value.