Answer:
25.8
Step-by-step explanation:
Let's write the reaction between magnesium-phosphide and potassium:
Mg3P2 + K = Mg + K3P
And now let's balance this equation:
Mg3P2+6K=3Mg+2K3P
We see that the ratio of magnesium-phosphide and potassium is 1:6, which means that for every mole of magnesium-phosphide there need to be 6 moles of potassium.
Since we have 4.3 moles of Mg3P2, there need to be 6 • 4.3 = 25.8 moles of potassium.