Calculate the weight of Lithium in a tablet containing 250 mg Lithium Carbonate. (Molecular Weight of Li2CO3 = 74; Atomic Weight of Li+ = 7)单项选择题
A
a. 47.3 mg
B
b. 94.6 mg
C
c. 74.6 mg
D
d. 56.6 mg
E
e. 250 mg

Question

Calculate the weight of Lithium in a tablet containing 250 mg Lithium Carbonate. (Molecular Weight of Li2CO3 = 74; Atomic Weight of Li+ = 7)

BlackTom AI · Accepted Answer

Fractional mass of Li in Li2CO3 = (2 × atomic weight of Li) / molecular weight of Li2CO3.

1. Relevant knowledge points and concepts
- Molar (molecular) mass and atomic mass allow composition by mass: mass fraction of an element = (sum of atomic masses of that element in the formula) / (molecular mass of the compound).
- Multiply the mass fraction by the sample mass to get mass of the element present.

2. Complete reasoning, step by step
- Atomic weight of Li = 7 (given). There are two Li atoms in Li2CO3, so total Li mass per formula unit = 2 × 7 = 14.
- Molecular weight of Li2CO3 = 74 (given).
- Mass fraction of Li in Li2CO3 = 14 / 74.

3. Calculations (all intermediate steps)
- Fraction = 14 / 74 = 0.189189... (repeating).
- Mass of Li in a 250 mg tablet = 250 mg × (14 / 74).
- 250 × 14 = 3500; 3500 / 74 = 47.297297... mg.
- Rounded to one decimal place = 47.3 mg.

4. Why the correct answer is correct
- The calculation directly applies percent composition: approximately 18.92% of lithium carbonate’s mass is lithium. 18.92% of 250 mg = ~47.3 mg. This matches option a.

5. Why the other options are incorrect
- b (94.6 mg) is exactly double the correct value (2 × 47.3 mg) and would correspond to incorrectly doubling the Li content.
- c (74.6 mg) is roughly 30% of 250 mg and does not follow the 14/74 fraction; it may reflect a mistaken use of different numbers.
- d (56.6 mg) does not match the calculated fraction (would correspond to a different atomic/molecular weight error).
- e (250 mg) assumes the tablet is pure lithium, which is incorrect because the tablet contains lithium carbonate, not elemental lithium.

Conclusion: The mass of lithium in a 250 mg Li2CO3 tablet is 47.3 mg, confirming option a.

Calculate the weight of Lithium in a tablet containing 250 mg Lithium Carbonate. (Molecular Weight of Li2CO3 = 74; Atomic Weight of Li+ = 7)单项选择题

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