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Chemistry Question 127 – AP-EAMCET 2026

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Chlorophyll contains 2.4% of magnesium. The number of magnesium atoms present in 2.0 g of chlorophyll is (N=6×1023 mol−1, At.wt of Mg=24 u)

First, determine the total mass of magnesium present in the given sample of chlorophyll using its percentage composition.

Step 1: Calculate the mass of Magnesium in the chlorophyll sample✦ Active

The question states that chlorophyll contains 2.4% magnesium by mass. We have 2.0 g of chlorophyll. To find the mass of magnesium in this sample, we use the percentage composition:

Mass of Mg=2.4100×2.0 g=0.024×2.0 g=0.048 g
💡 Teacher's Secret Hint

Remember to convert percentage to a decimal or fraction before multiplying.

Step 2: Calculate the number of moles of Magnesium○ Expand

The atomic weight of magnesium is given as 24 u, which implies its molar mass is 24 g/mol. Now, we can find the number of moles of magnesium using its mass and molar mass:

Moles of Mg=Mass of MgMolar mass of Mg=0.048 g24 g/mol=0.002 mol
💡 Teacher's Secret Hint

Always pay attention to units. Grams divided by grams/mole gives moles.

Step 3: Calculate the number of Magnesium atoms○ Expand

To convert moles to the number of atoms, we use Avogadro's number (NA=6×1023 mol−1):

Number of Mg atoms=Moles of Mg×NA Number of Mg atoms=0.002 mol×(6×1023 atoms/mol) Number of Mg atoms=(2×10−3)×(6×1023) Number of Mg atoms=12×1020 Number of Mg atoms=1.2×1021
💡 Teacher's Secret Hint

Be careful with scientific notation and power manipulation. A common mistake is 12×1020 being incorrectly written as 12×1021. Always ensure the coefficient is between 1 and 10 for standard scientific notation.

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