Final answer:
The percent mass of fluorine in the compound formed when 1g of hydrogen reacts with 19g of fluorine is 95%.
Step-by-step explanation:
The percent mass of an element in a compound is calculated by the formula (mass of element in the compound / total mass of the compound) × 100%. In this case, to find the percent mass of fluorine in the compound formed when 1g of hydrogen reacts completely with 19g of fluorine, we add the masses of hydrogen and fluorine to get the total mass of the compound, which is 1g + 19g = 20g. Then we take the mass of fluorine (19g) and divide it by the total mass of the compound (20g), and multiply by 100% to get the percent mass of fluorine in the compound.
So, the calculation would be (19g / 20g) × 100%, which equals 95%. Therefore, the percent mass of fluorine in the compound is 95%.