Hi everyone.
Could anyone help me with some MCQ questions?
1. Which of the following acids (together with their sodium salts) is the best choice to buffer a solution at pH 5.0?A. chloroacetic acid (Ka = 1.4 x 10
-3)
B. propanoic acid (Ka = 1.3 x 10
-5)
C. benzoic acid (Ka = 6.4 x 10
-5)
D. hypochlorous acid (Ka = 3.5 x 10
-8)
E. carbonic acid (Ka = 4.3 x 10
-7)
I've subsequently discovered that you solve this by saying pH = pKa, and pKa = -log10Ka, but I don't know where this comes from and if it always holds true? So I can solve this problem, I just don't know why we use pH = pKa.2. Calcium carbonate, CaCO3, has a solubility product, Ksp, of 2.8 × 10-9. What is the concentration of Ca2+ ions in a saturated solution of CaCO3?A. 1.4x10
-9 M
B. 2.8x10
-9 M
C. 5.6x10
-9 M
D. 5.3x10
-5 M
E. 1.1x10
-4 M
Is this just because at equilibrium, stoichiometry says that, [Ca2+] = [CO32-] = √Ksp ? Or is there something more to it?3. Iodide reacts with peroxodisulphate in aqueous solution to produce iodine and sulphate according to the equation:
2I-(aq) + S2O82-(aq) → I2(aq) + 2SO42-(aq)
If the rate at which I2 is produced is 2.0 x 10-6 Ms-1, then:A. S
2O
82- is consumed at 1.0 x 10
-6 Ms-1
B. I- is consumed at 2.0 x 10
-6 Ms-1
C. SO
42- is formed at 2.0 x 10
-6 Ms-1
D. S
2O
82- is consumed at 2.0 x 10
-6 Ms-1
E. SO
42- is formed at 1.0 x 10
-6 Ms-1
The provided answer says D, which makes sense, but I don't know why E isn't also true (by stoichiometric ratios)? i.e. D is true because the stoichiometric ratios of I2 and S2O82- are 1:1, but isn't E also true because SO42- will form at half the rate of I2 because it is producing twice the amount?4. Samples of chlorine, neon and nitrogen are contained in three identical 1litre flasks. All are at the same temperature. The mass of gas in each flask is the same. Which of the following is TRUE?A. They are all at the same pressure.
B. The density of chlorine is higher than the other two.
C. The density of nitrogen is lower than the other two.
D. The number of Ne atoms is half the number of Cl atoms. The pressure is highest in the flask of neon.
E. The pressure is highest in the flask of neon.
I got the answer E, because of the following rationale, but the provided answer said B?
Atomic masses: N < Ne < Cl
Molecular mass of gas: Ne < N2 < Cl2
Therefore Ne flask has most number of molecules and Cl2 has the least (to make the same mass).- A is false because n is different for each flask.
B is false because Cl2 has fewest molecules, thus is LEAST dense.
C is false because N2 is in the middle.
D is false because for 1 gram mass, n(Ne) = 0.05 mol and n(Cl) = 0.028 mol.
E is true because Ne contains the most molecules, maximizing PV=nRT.
So why is B the answer?Thanks everybody