Your reaction needed
So, start with the balanced chemical equation
Keep an eye out for the mole ratios that exist between iron and hydrochloric acid on one hand, and between hydrochloric acid and hydrogen gas on the other.
To solve this problem you have to work backwards from the amount of hydrogen gas produced, to the amount of iron required by using the number of moles of hydrochloric acid.
Since you know its concentration and volume, you can determine how many moles of
In order to see whether or not all the moles of hydrochloric acid reacted, you must look at how many moles of hydrogen gas were produced. SInce you're at STP, use the fact that 1 mole of any ideal gas occupies 22.7 L under trese conditions - this is known as the molar volume of a gas at STP (100 kPa, 273.15 K)
This means that not all the hydrochloric acid reacted, which implies that iron is a limiting reagent, i.e. it dictated how much of the hydrochloric acid actually reacted.
Since you know how much
Use iron's molar mass to calculate the mass needed
If you round this to two sig figs, the number of sig figs in 0.50 M. the answer will be