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Chemistry

Percent concentration Question 1 How many grams of CuSO4.5Н2O are necessary in order to prepare 902 g 16 w/w% CuSO4 solution (by mass)? 1 𝑚𝑜𝑙 𝐶𝑢𝑆𝑂4 𝑢𝑛𝑑 5 𝑚𝑜𝑙 𝐻2 𝑂 𝑚𝑠𝑜𝑙𝑢𝑡𝑒

𝑤 % = 𝑤 ∙ 𝑚𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 100

𝑚𝑠𝑜𝑙𝑢𝑡𝑒 =

16 ∙ 902𝑔 100

𝑚𝑠𝑜𝑙𝑢𝑡𝑒 = 144,32 𝑔 There are 144,32g solute in 902g 16%(w/w) solution. Calculate the molar mass of the pure substance and the molar mass of the crystallohydrate. 𝑀(𝐶𝑢𝑆𝑂4 ) = 159,57 𝑀(𝐻2 𝑂) = 18

𝑔 𝑚𝑜𝑙

𝑔 𝑚𝑜𝑙

𝑀(𝐶𝑢𝑆𝑂4 ∙ 5𝐻2 𝑂) = 249,60

𝑔 𝑚𝑜𝑙

Determine the mass 249,60 ∙ 144,32 = 𝟐𝟐𝟓. 𝟖𝒈 159,57

Question 2 How many grams of CuSO4.5Н2O are necessary in order to prepare 272 g 4 w/w% CuSO4 solution (by mass)? ➢ 17.0 g

Question 3 How many grams of CuSO4.5Н2O are necessary in order to prepare 898 g 5 w/w% CuSO4 solution (by mass)? ➢ 70.2

Question 4 Determine the w/w% concentration of a solution, prepared by 29 g of solute and 76 g solvent. 𝑤 𝑚𝑠𝑜𝑙𝑢𝑡𝑒 %= ∗ 100 𝑚𝑠𝑜𝑙𝑢𝑡𝑒 + 𝑚𝑠𝑜𝑙𝑣𝑒𝑛𝑡 𝑤 29 𝑤 %= = 𝟐𝟕. 𝟔 𝑤 29 + 76

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Question 5 How many grams NaCl are necessary in order to prepare 1,000 g 12 w/w% solution of NaCl (by mass)? 𝑚𝑠𝑜𝑙𝑢𝑡𝑒 =

12 ∙ 1000 = 𝟏𝟐𝟎𝒈 100

Question 6 How many grams NaCl are necessary in order to prepare 740 g 17 w/w% solution of NaCl (by mass)? 𝑚𝑠𝑜𝑙𝑢𝑡𝑒 =

17 ∙ 740 = 𝟏𝟐𝟓. 𝟖 100

Question 7 How many grams NaCl are necessary in order to prepare 220 g 15 w/w% solution of NaCl (by mass)? 𝑚𝑠𝑜𝑙𝑢𝑡𝑒 =

15 ∙ 220 = 𝟑𝟑 100

Question 8 Determine the w/w% concentration of FeSO4 solution, prepared by 63 g FeSO4.7H2O and 676 g H2O 𝑀(𝐹𝑒𝑆𝑂4 ) = 151,57

𝑔 𝑚𝑜𝑙

𝑀(𝐹𝑒𝑆𝑂4 ∙ 7𝐻2 𝑂) = 277,87 𝑛(𝐹𝑒𝑆𝑂4 ) =

𝑔 𝑚𝑜𝑙

63 𝑔 𝑚(𝐹𝑒𝑆𝑂4 ) = = 0,2267𝑚𝑜𝑙 𝑀(𝐹𝑒𝑆𝑂4 ∙ 7𝐻2 𝑂) 277,87 𝑔 𝑚𝑜𝑙

𝑚(𝐹𝑒𝑆𝑂4 ) = 𝑛(𝐹𝑒𝑆𝑂4 ) ∙ 𝑀(𝐹𝑒𝑆𝑂4 ) = 0,2267𝑚𝑜𝑙 ∗ 151,57

𝑔 = 34,4 𝑚𝑜𝑙

Berechnung w/w%: 𝑤 𝑚𝑠𝑜𝑙𝑢𝑡𝑒 ∗ 100 %= 𝑚𝑠𝑜𝑙𝑢𝑡𝑒 + 𝑚𝑠𝑜𝑙𝑣𝑒𝑛𝑡 𝑤 34,4 𝑤 ∗ 100 = 𝟒. 𝟕 % %= 63 + 676 𝑤

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Molarity Question 1 Calculate the molarity (CM) of an aqueous NaOH solution containing 10 g NaOH in 625 cm3 of solution. 625𝑐𝑚 3 = 0,625𝑙 𝑛(𝑁𝑎𝑂𝐻) =

𝐶𝑀 =

10 𝑚 = = 0,25𝑚𝑜𝑙 𝑀 39,99

𝑛𝑆𝑢𝑏𝑠𝑡𝑎𝑛𝑐𝑒 0,25 𝑚𝑜𝑙 𝑚𝑜𝑙 = = 𝟎. 𝟒 𝑉𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 𝑙 0,625 𝑙

Question 2 Calculate the grams of solute present in 1,800 cm3 of 0.3 M CaCl2 aqueous solution. 1800𝑐𝑚 3 = 1,8 𝑙 𝑛=𝑉∙𝑐 𝑛 = 1,8 𝑙 ∙ 0,3

𝑚𝑜𝑙 = 0,54 𝑚𝑜𝑙 𝑙

𝑚 = 𝑛 ∗ 𝑀 = 0,54 𝑚𝑜𝑙 ∗ 110,98

𝑔 = 𝟓𝟗. 𝟗 𝑚𝑜𝑙

Question 3 Calculate the molarity (CM) of an aqueous NaOH solution containing 20 g NaOH in 850 cm3 of solution. ➢ 0.6

Question 4 Calculate the grams of solute present in 400 cm3 of 0.6 M CaCl2 aqueous solution. ➢ 26.6

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pH Strong electrolytes Question 1 Calculate the рН of 0.05 M HCl solution 𝑝𝐻 = − log[𝐻3 𝑂+ ] 𝑝𝐻 = − log(0,05) = 𝟏. 𝟑

Question 2 Calculate the рН of 0.1 M HNO3 solution 𝑝𝐻 = − log(0,1) = 𝟏

Question 3 Calculate the pH of 0.005 M KOH solution 𝑝𝑂𝐻 = − log[𝑂𝐻 − ] 𝑝𝑂𝐻 = − log[0,005] = 2,3 𝑝𝐻 = 14 − 2,3 = 𝟏𝟏. 𝟕 Question 4 Calculate the рН of 0.0004 M HNO3 solution 𝑝𝐻 = − log(0,0004) = 𝟑. 𝟒

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pH_weak electrolytes Question 1 Calculate the рН of 0.001 M HCOOH solution (pKa = 3.75) 𝑝𝐶𝑎 = −log(𝐶𝑎 ) 𝑝𝐻 =

𝑝𝐻 =

𝑝𝐾𝑎 + 𝑝𝐶𝑎 2

3,75 + 3 = 𝟑. 𝟒 2

Question 2 Calculate the рН of 0.003 M NH4OH solution (pKb = 4.75) 𝑝𝐶𝑏 = −log(𝐶𝑏 ) 𝑝𝑂𝐻 =

𝑝𝑂𝐻 =

𝑝𝐾𝑏 + 𝑝𝐶𝑏 2

4,75 + 2,52 = 3,64 2

𝑝𝐻 = 14 − 3,64 = 𝟏𝟎. 𝟒

Question 3 Calculate the рН of 0.01 M NH4OH solution (pKb = 4.75) ➢ 10.6 Question 4 Calculate the рН of 0.0002 M HCOOH solution (pKa = 3.75) ➢ 3.7

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Buffers Question 1 Buffering capacity is expressed mathematically as follows 𝛥𝐶𝑎𝑐𝑖𝑑 o ß = 𝛥𝑝𝐻 𝛥𝐻 + ,𝑂𝐻 − 𝛥𝑝𝐻 𝛥𝐶𝑏𝑎𝑠𝑒 − 𝛥𝑝𝐻

✓ ß=± o

ß=

o

All answers are correct

Question 2 A solution that contains a weak acid and its conjugated base in roughly equal concentrations is o neutral ✓ a buffer o neither acidic or basic o a heterogeneous mixture Question 3 Which of the following is NOT characteristic of an effective buffer? o the acid-base concentrations are relatively high o there are roughly equal concentrations of conjugate acid and conjugate base ✓ the pH is always close to 7 o the pH of the buffered solution is within 1.0 of the pKa of the conjugate acid of the buffer Question 4 If a buffer solution of acetic acid – acetate ion (pH=4,74) had equal moles of HCl and NaOH added to it, what would the pH of the resulting solution be? ✓ pH = 4.74 o pH < 4.74 o pH cannot be determined o pH > 4.74 Question 5 What reaction occurs as a hydrochloric acid solution is added to a solution containing equal concentrations of acetic acid and sodium acetate? ✓ 𝐶𝐻3 𝐶𝑂𝑂− + 𝐻+ → 𝐶𝐻3 𝐶𝑂𝑂𝐻 o 𝐶𝐻3 𝐶𝑂𝑂𝐻 + 𝐻+ → 𝐶𝐻3 𝐶𝑂 + + 𝐻2 𝑂 o 𝐶𝐻3 𝐶𝑂𝑂𝐻 + 𝐻+ → 𝐶𝐻3 𝐶𝑂𝑂𝐻2+ o 𝐶𝐻3 𝐶𝑂𝑂𝐻 + 𝐻𝐶𝑙 → 𝐶𝐻3 𝐶𝑂𝑂− + 𝐻2 𝐶𝑙+ Question 6 What is the assumption that the Hendersen-Hasselbach equation makes? o the difference in pH from pKa of the conjugate acid is small o the concentrations of the acid-base species are large enough to be an effective buffer over small additions of acid or base o the Ka is small enough that the amount of dissociation is negligible for calculation purposes ✓ all of the above + The buffer depends on the ratio of both buffer components, if their concentration is equal the pH=pKa

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Question 7 Calculate the pH of a solution that is 0.30 M in ammonia (NH3) and 0.20 M in ammonium chloride (NH4Cl), pKb=4.75 0.2 = 4,57 0.3 𝑝𝐻 = 14 − 4,57 = 9,43

𝑝𝑂𝐻 = 4.75 + 𝑙𝑜𝑔

Question 8 Consider a solution initially containing 0.40 mol fluoride anion and 0.30 mol of hydrogen fluoride (HF). What is the pH after addition of 0.20 mol of HCl to this solution? (pKa=3.14)

𝑝𝐻 = 3,14 + 𝑙𝑜𝑔

(0,2 − 0,4) = 2,74 (0,2 + 0,3)

Question 9 Calculate the pH of a buffer solution prepared by mixing 30 cm3 0.25M NH4Cl and 20 cm3 0.4M NH4OH /pKb (NH4OH) = 4.75 30 ∗ 0.25 = 4,72 20 ∗ 0.4 𝑝𝐻 = 14 − 4.72 = 9.28

𝑝𝑂𝐻 = 4.75 + 𝑙𝑜𝑔

Question 10 Calculate the pH of a buffer solution that contains 75 cm3 0.2M of CH3COOH and 25 cm3 0.6M of CH3COONa /pKa (CH3COOH) = 4.76 𝑝𝐻 = 4.76 +

25 ∗ 0.6 = 4.76 75 ∗ 0.2

Question 11 Calculate the pH of a buffer solution prepared by mixing 20 cm3 0.25M NH4Cl and 20 cm3 0.4M NH4OH /pKb (NH4OH) = 4.75 ➢ 9,46 Question 12 Which of the following is a conjugated acid-base pair? ➢ NH3 and NH4+ Question 13 Which of the following is not a conjugate acid-base pair? ➢ HNO3 and HNO2 Question 14 What is the pH of a buffer solution where [HA]=[A-] ➢ pH=pKa

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Question 15 Three acids found in foods are lactic acids, oxalic acid and magic acid. The pKa values are: LA: 3,88 OA: 1,23 MA: 3,40 Which list has these acids in order of decreasing acid strength ➢ OA > LA > MA Question 16 Identify the two sets of conjugate acid-base pairs in the following acid-base reaction 𝐻𝐵𝑟 + 𝐻2 𝑂 → 𝐵𝑟 − + 𝐻3 𝑂+ ➢ HBr/Br- and H2O/H3O+ Question 17 Ammonia acts as a weak base in aqueous solution. What is the acid that reacts with this base when ammonia is dissolved ➢ none, there are no acids in pure water 𝑁𝐻3 + 𝐻2 𝑂 → 𝑁𝐻4+ + 𝑂𝐻−

Question 18 What is the formula for determining [OH-] of a solution when given the concentration of hydronium ions [H3O+]. 𝐾 ➢ [𝑂𝐻− ] = [𝐻 𝑊 𝑂+] 3

Question 19 Which of the following is a conjugate acid-base pair ➢ HI and IQuestion 20 Which of the following is not a conjugate acid-base pair? ➢ H3O+ and OHQuestion 21 Which of the following can be mixed together in water to produce a buffer solution? ➢ Na2HPO4 and NaH2PO4 Question 22 How many times more acidic is a solution with a pH of 3 compared to absolution with a pH of 6? ➢ 1000 Question 23 What reaction occurs as a hydrochloric acid solution is added to a solution containing equal concentrations of acetic acid and sodium acetate? ➢ CH3COO- + H+ → CH3COOH

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Other Questions Which of the following is amphoteric? ➢ H2O What are the products of neutralization reaction? ➢ H2O and ionic salt The key characteristics of redox reduction is the ➢ passing of one or more electrons from one species to another

Essay Explain how a buffer solution manages to stabilize the pH against the addition of acid and base. ➢ Buffers maintain the pH of a solution by adjusting the direction of their chemical reactions (dissociating or re-associating) in response to increase or decrease in H+ ion concentration that can be caused by other substances entering or exiting the solution ➢ In case of addition of an acid or base the buffer either accepts of donates a proton. This maintains constant pH

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