Consider the following nuclear reaction : 238U -> 234Th + 4He Take masses of 238U , 234Th and 4He as 238.050 u, 234.043 u and 4.003 u, respectively. The Q value for the reaction, in keV, is : [Given : 1 u = 931.5 MeV c-2]

The correct answer is Option 1. Full mathematical solution steps and hints are accessible in the interactive StemCET workspace.

Physics Q7 – NEET-UG 2026

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Home Physics NucleiQuestion #376

Consider the following nuclear reaction :

^{238} U \to^{234} Th +^{4} He

Take masses of

^{238} U

^{234} Th

and

^{4} He

238.050 u

234.043 u

and

4.003 u

, respectively. The Q value for the reaction, in keV, is : [Given :

1 u = 931.5 {MeV c}^{- 2}

]

Clue 1 of 3: Nuclear Reaction Energy

The Q-value of a nuclear reaction represents the energy released or absorbed during the reaction.

🧠 Full Solution Path

Step 1: Calculate the mass defect (

Δ m

) of the reaction.✦ Active

The mass defect is the difference between the total mass of the reactants and the total mass of the products. The given nuclear reaction is $^{238} U \to^{234} Th +^{4} He$ . Reactant mass: $m (^{238} U) = 238.050 u$ Product masses: $m (^{234} Th) = 234.043 u$ and $m (^{4} He) = 4.003 u$

Δ m = m (^{238} U) - (m (^{234} Th) + m (^{4} He))

Δ m = 238.050 u - (234.043 u + 4.003 u)

Δ m = 238.050 u - 238.046 u

Δ m = 0.004 u

Step 2: Calculate the Q-value in MeV.○ Expand

Using the given conversion factor $1 u = 931.5 {MeV c}^{- 2}$ , the Q-value (energy released) can be calculated from the mass defect.

Q = Δ m \times 931.5 MeV/u

= 0.004 \times 931.5 MeV

= 3.726 MeV

💡 Teacher's Secret Hint

Ensure to use the correct conversion factor for atomic mass units to energy.

Step 3: Convert the Q-value from MeV to keV.○ Expand

Since $1 MeV = 1000 keV$ , convert the Q-value to keV as required by the question.

Q = 3.726 \times 1000 keV

= 3726 keV

💡 Teacher's Secret Hint

Pay attention to the units required in the final answer (keV).

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Physics Question 7 – NEET-UG 2026

Clue 1 of 3: Nuclear Reaction Energy

🧠 Full Solution Path