Final answer:
To find the number of atoms in 12.5 g of (NH₄)₂Cr₂O₇, convert the mass to moles using the molar mass and then multiply by Avogadro's number and the number of atoms per formula unit.
Step-by-step explanation:
To determine how many atoms are in 12.5 g of (NH₄)₂Cr₂O₇, we must first find the molar mass of (NH₄)₂Cr₂O₇ and then use that to convert grams to moles. After finding the number of moles, we will then use Avogadro's number to convert moles to atoms.
The molar mass of (NH₄)₂Cr₂O₇ is calculated by adding the atomic masses of all atoms in the compound: (2 × ((14.01 for N) + (4 × 1.01 for H))) + (2 × 51.9961 for Cr) + (7 × 16.00 for O). This results in a molar mass of approximately 252.06 g/mol.
Next, we can convert the given mass into moles: 12.5 g ÷ 252.06 g/mol = 0.0496 moles of (NH₄)₂Cr₂O₇.
Since 1 mole of (NH₄)₂Cr₂O₇ contains 6.022 × 10²³ formula units, and each formula unit contains a total of 19 atoms (2 (NH₄)+ and 1 Cr₂O₇²-), we can then multiply: 0.0496 moles × 6.022 × 10²³ formula units/mole × 19 atoms/formula unit to get the total number of atoms.