We have to use Avogadro's number to find the number of moles of BaF2. This number says that there are 6.022 x 10^(23) molecules in 1 mol. The calculation would be:
![2.61\cdot10^(25)moleculesBaF_2\cdot(1molBaF_2)/(6.022\cdot10^(23)moleculesBaF_2)=43.34molesBaF_2.](https://img.qammunity.org/2023/formulas/chemistry/college/lkrvt04ew27tn2zei3y22f8csws4nen93p.png)
The answer is that we have 43.34 moles of BaF2 in 2.61 x 10^(25) molecules/formula units.