Given:
- Volume of K2Cr2O7 solution = 25 mL
- Concentration of K2Cr2O7 solution = 0.3 M

To Calculate:
- Mass of Fe3O4

Solution:

Step 1: Balance the equation:
The reaction between Fe3O4 and K2Cr2O7 can be represented by the following balanced equation:

8 Fe3O4 + 3 K2Cr2O7 + 7 H2SO4 → 8 Fe2(SO4)3 + 3 Cr2(SO4)3 + 3 K2SO4 + 7 H2O

Step 2: Calculate the moles of K2Cr2O7:
Given that the volume of the K2Cr2O7 solution is 25 mL and the concentration is 0.3 M, we can calculate the moles of K2Cr2O7 using the formula:

Moles = Concentration × Volume

Moles of K2Cr2O7 = 0.3 M × 0.025 L = 0.0075 moles

Step 3: Determine the stoichiometry:
From the balanced equation, we can see that the ratio between K2Cr2O7 and Fe3O4 is 3:8. Therefore, for every 3 moles of K2Cr2O7, we need 8 moles of Fe3O4.

Step 4: Calculate the moles of Fe3O4:
Using the stoichiometry, we can calculate the moles of Fe3O4:

Moles of Fe3O4 = (0.0075 moles K2Cr2O7) × (8 moles Fe3O4 / 3 moles K2Cr2O7) = 0.02 moles

Step 5: Calculate the mass of Fe3O4:
Finally, we can calculate the mass of Fe3O4 using its molar mass. The molar mass of Fe3O4 is calculated as:

Molar mass of Fe3O4 = (3 × Molar mass of Fe) + (4 × Molar mass of O)

Molar mass of Fe = 55.845 g/mol
Molar mass of O = 15.999 g/mol

Molar mass of Fe3O4 = (3 × 55.845 g/mol) + (4 × 15.999 g/mol) = 231.532 g/mol

Mass of Fe3O4 = (0.02 moles) × (231.532 g/mol) = 4.63 g

Answer:
The mass of Fe3O4 required to react completely with 25 mL of 0.3 M K2Cr2O7 is 4.63 grams.