设铁粉中纯铁的质量为x
Fe+2HCl═FeCl₂+H₂↑
56             2
x             0.2g

56

2

=

x

0.2g

  x=5.6g
铁粉中纯铁的质量分数=

5.6g

6.3g

×100%=88.9%
答：铁粉中纯铁的质量分数是88.9%．