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A Level Chemistry > 1.12 Acids & bases > Flashcards

1.12 Acids & bases Flashcards

1
Q

Define weak acid
ii) define a strong acid

A

A proton donor that
i) partially dissociates
ii) fully dissociates / completely ionises to give H^+ ions in water

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2
Q

Calculate the pH for a solution of HCl with a concentration of 0.064. Give your answer to 2 decimal places.

A

pH = -log{H^+}
= 1.19

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3
Q

Calculate the pH for a solution of NaOH with a concentration of 0.157. Give your answer to 2 decimal places.
Kw at 25 oC = 1 x 10-14 mol2 dm-6

A

Since NaOH is a strong base, it dissociates completely in water:
NaOH → Na^+ + OH^−
This means the concentration of hydroxide ions, is equal to the concentration of NaOH, which is 0.157 mol/dm³
calculate pOH=−log(0.157) = 0.803
pH+pOH=14(at25°C)
14-0.803 = 13.20

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4
Q

What is a Lowry Bronsted base

A

proton acceptor

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5
Q

Calculate the pH for a solution of Ba(OH)2 with a concentration of 0.21. Give your answer to 2 decimal places.
Kw at 25 oC = 1 x 10-14 mol2 dm-6
b) show method using Kw

A

Ba(OH) 2 →Ba ^2+ +2OH ^−
concentration of OH^- = 0.21 x 2 = 0.42
pOH = -log(0.42) = 0.376
pH = 14 - 0.376 = 13.62
b) Kw = {OH}^- {h}^+
sub values in, use - log to get answer of 13.62

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6
Q

What type of acids are CH3COOH & HCOOH

A

Weak acids

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7
Q

A beaker contains 100.0 cm–3 of 0.0125 mol dm−3 nitric acid.
Calculate the pH of the solution formed after 50.0 cm3 of 0.0108 mol dm−3 aqueous sodium hydroxide are added to the beaker.
Give your answer to 2 decimal places.

A

HNO 3(aq) + NaOH(aq)→NaNO3(aq)+H2O(l)
moles of HN03 = 0.1 x 0.0125 = 0.00125 mol
moles of NaOH = 0.0108 x 0.05 = 0.00054 mol
- NaOH is limiting reagent as moles of HNO3 left = 0.00125 - 0.00054 = 0.00071 mol
total volume = 0.1 + 0.05 = 0.15 dm^3
concentration = 0.00071 / 0.15 = 0.00473
pH = -log (0.00473) = 2.326 = 2.33

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8
Q

Define pH

A

pH = -log10{H^+}

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9
Q

Chemical equation for nitric acid + potassium oxide

A

2HNO3 (aq) + K2O (s) -> 2KNO3 (aq) + H2O (l)

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10
Q

Chemical equation for phosphoric acid + sodium hydroxide

A

H3PO4 (aq) + 3NaOH (s) -> Na3PO4 (aq) + 3H2O (l)

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11
Q

Suggest why the pH probe is washed with distilled water between each of the calibration measurements.

A

Different solutions must not contaminate each other /
to wash off any substance that could affect the reading

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12
Q

The calibrated pH meter is used to monitor the pH during a titration of hydrochloric acid with sodium hydroxide.
Explain why the volume of sodium hydroxide solution added between each pH measurement is smaller as the end point of the titration is approached.

A

To avoid missing the end point

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13
Q

state why a certain indicator will be suitable for a certain titration

A

all have a colour change within the vertical part of the titration curve

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14
Q

Explain why [H2O] is not shown in the Kw expression.

A

[H2O] is (almost) constant
or
[H2O] is (very) large in comparison to [H+] and [OH]^-

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15
Q

Suggest why the pH probe is washed with distilled water between each of the calibration measurements.

A

Different solutions must not contaminate each other
or
To wash off any residual solution/substance which could
interfere with the reading

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16
Q

The calibrated pH meter is used to monitor the pH during a titration of hydrochloric acid with sodium hydroxide.
Explain why the volume of sodium hydroxide solution added between each pH measurement is smaller as the end point of the titration is approached.

A

To avoid missing the end point
Very little pH change per cm3 added at start, large change in
pH near end point

17
Q

Which statement about pH is correct?
A The pH of a weak base is independent of
temperature.
B At temperatures above 298 K, the pH of pure
water is less than 7.
C The pH of 2.0 mol dm–3 nitric acid is
approximately 0.30
D The pH of 0.10 mol dm–3 sulfuric acid is greater
than that of 0.10 mol dm–3 hydrochloric acid.

A

B

17
Q

A 0.10 mol dm–3 aqueous solution of an acid is added slowly to 25 cm3 of a 0.10
mol dm–3 aqueous solution of a base.
Which acid–base pair has the highest pH at the equivalence point?
A CH3COOH and NaOH
B CH3COOH and NH3
C HCl and NaOH
D HCl and NH3

A

A

18
Q

A student is required to make 250 cm3 of an aqueous solution that contains
an accurately measured mass of sodium hydrogen sulfate (NaHSO4).
Describe the method that the student should use to make this solution.

A
  • weigh solid, before with just container and then after solution added and then subtract
  • dissolve in water in suitable container (not in 250cm^3 of water)
  • transfer with washings into 250cm^3 volumetric / graduated flask
  • make up to 250cm^3 mark and then shake
19
Q

(HSO4)^– ⇌ SO4^2– + H^+
Some sodium sulfate is dissolved in a sample
explain why this increases pH

A

equilibrium shifts left to oppose decrease in concentration in SO4^2-
So [H^+ decreases)

20
Q

Which indicator should be used in a titration to find the concentration of a
solution of methylamine using 0.010 mol dm–3 hydrochloric acid?
A Thymol blue (pH range 1.2–2.8)
B Bromophenol blue (pH range 3.0–4.6)
C Phenol red. (pH range 6.8–8.4)
D Phenolphthalein (pH range 8.3–10.0)

A

B

21
Q
A

B

22
Q
A

B

23
Q

Suggest briefly a practical procedure that a student could use to obtain
a curve for a pH - volume graph
b) The student was provided with samples of three different indicators.
Suggest how the practical procedure in part (a) could be refined by the
student to identify the most suitable indicator.

A
  • place a fixed volume of alkali in a flask or beaker
  • add acid in small portions from a burette
  • stir and use pH meter to record pH after each addition of acid
    b)
  • ## repeat experiment with each indicator
24
Q

Student was provided with 3 different indicators. Suggest how student could identify most suitable indicator.

A

Repeat experiment with each indicator
Select indicator that’s colour changes rapidly when pH changes from about 7 to 4

25
Q

A satellite orbiting the Earth moves to an orbit which is closer to the Earth.
what happens to speed of satellite and what happens to time for one orbit of earth

A

speed : increases
time : decreases

26
Q
A

Mass = moles x 68 = 0.339 grams

A Level Chemistry (13 decks)

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