Concepts of Acids and Bases II Quiz

Answer: Hydrochloric Acid

Weak Brønsted-Lowry acids have strong conjugate bases while weak Brønsted-Lowry bases have strong conjugate acids. And vice versa, weak acids have strong conjugate bases and weak bases have strong conjugate acids.

According to the Brønsted-Lowry theory, the criterion of a substance being an acid or a base is whether it loses or gains a proton during a reaction, respectively. Conjugate bases or conjugate acids are simply a proton added or removed to the acid or base respectively.

Answer: Potassium Methanoate and Methanoic Acid

Buffers are substances which resist change in their pH. There are two types of buffers - acidic and basic. Acidic buffers consist of a weak acid and the salt of its conjugate base while basic buffers consist of a weak base and the salt of its conjugate acid.

However, it is possible that ammonium hydroxide and hydrochloric acid can form a buffer solution provided that hydrochloric acid is present in half the amount as ammonium hydroxide. So can potassium hydroxide mixed in half the amount as methanoic acid. In each case, the constituents react to form the needed combination of weak base (ammonium hydroxide) and salt of its conjugate acid (ammonium chloride), or a weak acid (methanoic acid) and the salt of its conjugate base, potassium methanoate.

Answer: Indicators must be completely neutral substances

According to Ostwald's theory, indicators are themselves slightly acidic or basic.

The following equilibrium shows the activity of an indicator :-
HIn ⇌ H⁺ + In⁻

Indicators and their ions are colored differently as their bonds are transformed after giving up protons, such that differing wavelengths of light are absorbed by the electrons. When a base is added, the hydroxide ions released combine with the hydrogen ions to form water, reducing the amount of hydrogen ions and hence shifting equilibrium to the right. This increases the concentration of (In⁻) and its color is dominant.

When an acid is added, the equilibrium shifts to the left to use up the excess hydrogen ions added then, and this increases the concentration of (HIn).

Answer: Forward

Notice that it is not mentioned that the given concentrations are at equilibrium. In order to determine whether or not they are at equilibrium, we need to find the concentration/reaction quotient, which is the product of the concentrations of the products divided by that of the reactants, to the power of their respective reaction coefficients.

If the concentration quotient is the same as the value of the equilibrium constant, then the reaction is at equilibrium and so, the net reaction will not proceed anywhere.

Upon calculation, it is found out that the concentration quotient is just ([Z]^2/[X]*[Y]= 2^2/1*1=) 4. Since the value is less than the equilibrium constant, the concentration of products needs to increase, i.e. the net reaction shifts forward.

You can also calculate the final concentrations using the ICE*, or Initial-Change-Equilibrium method:-

Our initial concentrations are given in the question - the respective concentrations of X,Y and Z are 1, 1 and 2 molars respectively.

Let the concentration of moles of X or Y reacted be 'n'. Then the concentration moles of Z added would be 2n.

The equilibrium concentrations of X,Y and Z would be (1-n), (1-n) and (2+2n) respectively.

The equilibrium expression would be :-
Kc = (Concentration of Z)^2 / [ (1-n)*(1-n) ]
5 = (2+2n)^2 / [ (1-n)*(1-n)]

You will get a quadratic equation. Quadratic equations give two values for n, pick the value that is consistent. (Hint : If you pick the greater value of n, (1-n) would be negative, which isn't possible.)

You will get approximately n=0.06, and the equilibrium concentrations of X, Y and Z would be 0.94, 0.94 and 2.12 molars respectively.

*This isn't an official term, but simply a memory mnemonic to help you remember the steps.

Answer: 1%

If the pKa is 6.0, then the value of Ka (or the acid disassociation constant) is the negative log of 6.0, or 10^-6.

Given the assumption, and following Ostwald's theory of dilution, the amount of concentration times the degree of disassociation squared equals the acid disassociation constant. If you plug in the values given the question, the degree of disassociation will be found to be 0.01.

Note that a percentage was requested in the question, in which case you will have to multiply this value by 100%.

The assumption given simply makes our calculations easier, otherwise we may have to solve quadratic equations with exponentially small numbers. Given that the disassociation is just 1%, that was a fair assumption. Generally, it is recommended to perform the calculation without the assumption if the value exceeds 5%.

Answer: 1 kilogram of NaOH added to 50 L of 1 M HCl solution

10 L of 0.1 Molars NaOH is going to mean 1 moles of NaOH. ( Volume *Concentration = amount in moles). This would completely neutralize the HCl and the resultant pH would be 7.

When 2 moles of phosphoric acid are added to 10 L of 0.15 M NaOH, 0.05 moles of the acid would remain but since phosphoric acid is a weak acid, the decrease in pH will be very low.

1 kilogram of NaOH means 40 moles of NaOH. (Mass given/Molar mass = Amount in moles). 50 L of 1M HCl would mean 50 moles of HCl. This gives you 10 moles of HCl in 50 L, i.e. 0.2 M hydrogen ion concentration and a very low pH.

150 L of 0.01 M NaOH added to 50 L of 0.4 M HCl would contain 18.5 excess moles in a 200 L solution. That is a hydrogen ion concentration of 0.0925 M, giving you a higher pH than the correct choice.

Answer: True

In contrast to the Arrhenius concept or Bronsted-Lowry theory, whose criterion for classification requires that acids disassociate or donate hydrogen ions, any substance that accepts an electron pair is said to be a Lewis acid.

An example of a Lewis acid without hydrogen in its chemical formula would be boron trifluoride, (BF3). The boron atom in the middle is electron-deficient, having just six electrons in its outer shell. Therefore it can accept a pair of electrons to finish the octet.

Answer: The equivalence point is higher

The equivalence point will be higher since the salt produced is slightly basic, meaning a higher pH. The initial pH is going to be the same, since we are using the same solution of the strong base. There is a point of inflection, although the 'jump' in pH is lower.

Another difference would be that a 'buffer region' would be present, as weak acids release fewer hydrogen ions and hence do not react as readily as strong acids.

Answer: Magnesium Chloride

Hydrolysis occurs when the ions of the salt combine with either hydrogen or hydroxide ions in water to form their parent (weak) acid or base. Hydrolysis is the reason salts may not be completely neutral.

Magnesium chloride is the salt of a strong acid (hydrochloric acid) and a strong base (magnesium hydroxide), so its ions remain completely disassociated in water.

Answer: Brønsted-Lowry acids release hydrogen ions upon disassociation in water

It is mentioned that we are concerned with Brønsted-Lowry theory, which classifies acids as donors of hydrogen ions/protons. One can safely assume that they may need to contain hydrogen then. However, Lewis acids need not contain hydrogen.

The equivalence point, i.e. the pH after neutralization is not necessarily 7. The salt produced may be slightly alkaline or acidic, if either the acid or the base reacting is not strong. (or if the respective ionization constants of the weak acid and weak base are unequal).

The product of the acid ionization constant and the ionization constant of its conjugate base is the ionic product of water at that temperature. The concentration of hydrogen ions in water is indeed 10^-7 molars, but the degree of disassociation (and hence the hydrogen ion concentration) increases with increase in temperature and decreases with decrease in temperature.