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📝 Molecular weight vs molecular mass daltons kDa (14 MCQs)

📖 From Principles of Biochemistry • 1. The Foundations of Biochemistry • 14 questions available

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What is Molecular weight vs molecular mass daltons kDa?

Definition:
Molecular weight and molecular mass are often used interchangeably, but molecular weight (MW) refers to the ratio of the mass of a molecule to 1/12 of the mass of carbon-12, while molecular mass is the actual mass in atomic mass units (Da) or daltons, with 1 Da = 1 g/mol, and for large biomolecules, kilodaltons (kDa) are commonly used, where 1 kDa = 1000 Da.

Working:
These terms are used to describe the mass of molecules, calculated by summing the atomic masses of all atoms in the molecule, and in biochemistry, they are essential for determining concentrations (e.g., molarity using $\text{Molarity} = \frac{\text{mass (g)}}{\text{molecular weight (g/mol)} \times \text{volume (L)}}$ ), and for techniques like gel electrophoresis and mass spectrometry to identify and characterize proteins and nucleic acids.

Example:
A simple example is water (H₂O), with a molecular mass of 18 Da (oxygen 16 + 2 hydrogen 1 each), and a protein like hemoglobin has a molecular weight of about 64,500 Da (64.5 kDa), while a small peptide might be 2 kDa, illustrating the range of molecular sizes in biology.

Reason:
Understanding molecular weight and mass is fundamental for quantitative biochemistry, as it is necessary for preparing solutions, interpreting experimental data, and understanding protein structure and function, and it is a key concept in analytical and pharmaceutical chemistry.

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📝 All Molecular weight vs molecular mass daltons kDa MCQs

Q1. A protein subunit has a molecular mass of 42,000 Da. Which value expresses the same mass most conveniently for biochemical calculations?

A.42 kDa ✅

B.420 kDa

C.4.2 kDa

D.0.42 kDa

💡 Difficulty: easy | ✅ Correct: A

📖 Explanation: A kilodalton equals 1,000 daltons. Therefore 42,000 Da divided by 1,000 gives 42 kDa. The numerical value changes because the unit changes, but the physical molecular mass remains exactly the same.

Q2. A researcher reports that a DNA fragment has a molecular mass of 3.2 kDa and also describes it as having a molecular weight of 3,200 Da. What is the best interpretation?

A.The two reported values are numerically inconsistent

B.The values represent the same molecular mass using different units ✅

C.The second value must be 3.2 atomic mass units

D.Molecular weight must always be expressed in kilograms

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: The numerical values differ by a factor of 1,000 because kDa and Da are different units. Thus 3.2 kDa equals 3,200 Da. In biochemical contexts molecular mass is appropriately expressed in Da or kDa.

Q3. A molecule contains 150 atoms whose average atomic masses produce a total of approximately 18,000 u. Which conversion gives its approximate molecular mass in kilodaltons?

A.0.018 kDa

B.1.8 kDa

C.18 kDa ✅

D.180 kDa

💡 Difficulty: medium | ✅ Correct: C

📖 Explanation: A molecular mass of 18,000 u corresponds numerically to approximately 18,000 Da because the atomic mass unit and dalton are equivalent for this purpose. Dividing by 1,000 gives approximately 18 kDa.

Q4. A student argues that a molecule with a mass of 60 kDa must physically weigh 60,000 kilograms because kilo means 1,000. Which correction is most scientifically appropriate?

A.60 kDa means 60,000 Da not 60,000 kg ✅

B.60 kDa means 60 kg

C.A dalton is exactly the same as a gram

D.Kilodaltons are used only for molecular volume

💡 Difficulty: easy | ✅ Correct: A

📖 Explanation: The prefix kilo indicates a factor of 1,000 within the dalton unit. Therefore 60 kDa equals 60,000 Da. A dalton is an extremely small mass unit and is not interchangeable with kilograms or grams.

Q5. A laboratory instrument estimates an enzyme mass as 95,500 Da. A report requires the value in kDa rounded to one decimal place. Which result should be entered?

A.9.6 kDa

B.95.5 kDa ✅

C.955 kDa

D.0.0955 kDa

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: Converting daltons to kilodaltons requires division by 1,000. Thus 95,500 Da becomes 95.5 kDa. Because the requested precision is one decimal place and the conversion is exact at this stage the reported value remains 95.5 kDa.

Q6. Two proteins are measured as 48 kDa and 0.072 MDa respectively. Which conclusion is correct if $1\,\mathrm{MDa}=1000\,\mathrm{kDa}$ ?

A.The first protein is larger by 24 kDa

B.The second protein is larger by 24 kDa ✅

C.Both proteins have the same mass

D.The second protein is larger by 72 kDa

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: Converting 0.072 MDa to kilodaltons gives $0.072\times1000=72$ kDa. Comparing 72 kDa with 48 kDa shows that the second protein is larger by 24 kDa.

Q7. A researcher compares two forms of the same protein. Form X is reported as 72,000 Da while Form Y is reported as 68 kDa. Which statement correctly compares them?

A.Form X is 4 kDa heavier ✅

B.Form Y is 4 kDa heavier

C.They have identical masses

D.The comparison is impossible because Da and kDa measure different physical quantities

💡 Difficulty: medium | ✅ Correct: A

📖 Explanation: Converting Form X gives $72,000\,\mathrm{Da}=72\,\mathrm{kDa}$ . Form Y is 68 kDa. Therefore Form X exceeds Form Y by $72-68=4$ kDa. Different units do not prevent comparison after conversion.

Q8. A student's calculation starts with $2.5\times10^4$ Da and concludes that the molecular mass is 250 kDa. Which step contains the error?

A.The student multiplied by 10 instead of dividing by 1,000 ✅

B.The student should have converted Da into atomic mass units first

C.The student should have added 1,000 instead of dividing

D.There is no error

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: Since $1\,\mathrm{kDa}=1000\,\mathrm{Da}$ , converting $25,000$ Da to kDa requires division by 1,000. The correct result is 25 kDa, so 250 kDa is ten times too large.

Q9. A mass spectrometry calibration plot shows measured molecular mass on the vertical axis and calibration-standard mass on the horizontal axis. The points closely follow $y=x$ . A sample gives 75 on the instrument scale. What is the best interpretation if the axes are both in kDa?

A.The sample has approximately 7.5 kDa

B.The sample has approximately 75 kDa ✅

C.The sample has approximately 750 Da

D.The sample has approximately 75,000 kDa

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: A calibration relationship close to $y=x$ indicates that the measured value corresponds closely to the known molecular mass. Since both axes are labeled in kDa the sample measurement of 75 corresponds to approximately 75 kDa.

Q10. A scientist receives two database entries for a protein: 52,000 Da and 52 kDa. Before concluding that the database contains conflicting molecular masses the scientist should first:

A.Convert both values to the same unit ✅

B.Assume the larger numerical value is the larger molecule

C.Convert both values into kilograms

D.Treat daltons as a volume unit

💡 Difficulty: easy | ✅ Correct: A

📖 Explanation: The two entries are equivalent because $52\,\mathrm{kDa}=52,000\,\mathrm{Da}$ . Converting measurements to a common unit is essential before comparing numerical values because different prefixes change the numerical representation.

Q11. A peptide contains components whose approximate masses sum to 7,500 u. During a report review one researcher writes 7,500 kDa while another writes 7.5 kDa. Which report is consistent with the calculated mass?

A.7,500 kDa

B.75 kDa

C.7.5 kDa ✅

D.0.0075 kDa

💡 Difficulty: medium | ✅ Correct: C

📖 Explanation: The calculated mass is approximately 7,500 u or equivalently 7,500 Da. Converting to kilodaltons gives $7,500/1,000=7.5$ kDa. The 7,500 kDa value would be 1,000 times too large.

Q12. A student says that because a molecule has a molecular mass of 18,000 Da it must contain exactly 18,000 atoms. Why is this reasoning invalid?

A.Every atom has a mass of exactly 1 Da

B.The dalton measures molecular mass while atom counts depend on the masses of the constituent atoms ✅

C.Molecular mass is measured only in kilograms

D.Atoms cannot contribute to molecular mass

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: A dalton is a unit of mass based on atomic-scale mass rather than a count of atoms. Different atoms contribute different approximate masses, so a molecule's numerical mass cannot directly be interpreted as its number of atoms.

Q13. Two laboratories report the same antibody as 150 kDa and 150,000 Da. Laboratory A claims its antibody is 1,000 times heavier because 150,000 is numerically larger. Which reasoning best resolves the disagreement?

A.Laboratory A is correct because larger numbers always indicate greater mass

B.Laboratory B is correct because kDa is a smaller unit

C.The reports represent the same mass because

150\,\mathrm{kDa}=150,000\,\mathrm{Da}

✅

D.Neither laboratory can use daltons for proteins

💡 Difficulty: hard | ✅ Correct: C

📖 Explanation: Numerical magnitude cannot be compared without considering units. Since $1\,\mathrm{kDa}=1,000\,\mathrm{Da}$ , multiplying 150 kDa by 1,000 gives 150,000 Da. Therefore both laboratories report the same molecular mass.

Q14. A theoretical molecular assembly has four identical subunits each with a mass of 12.5 kDa. If the complete assembly contains all four subunits and no additional mass is considered, what is its molecular mass in daltons?

A.50 Da

B.500 Da

C.50,000 Da ✅

D.500,000 Da

💡 Difficulty: hard | ✅ Correct: C

📖 Explanation: Each subunit has a mass of 12.5 kDa. Four subunits therefore give $4\times12.5=50$ kDa. Converting to daltons gives $50\times1000=50,000$ Da, requiring two conversion steps.

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