Chemistry "Mixed" Problem Help Please!?

Question

1- [Conservation of Mass Problem]
Some iron wire weighing 5.6g is placed in a beaker & covered with 15.0g of dilute hydrochloric acid.The acid reacts w/ the metal & gives off hydrogen gas,which escapes into the surrounding air. After reaction the contents of the beaker weighs 20.4g.What's the mass of Hydrogen gas produced by the reaction?

2- A solid cylinder of iron of circular cross section w/ a radius of 1.500cm has a ruler etched along its length.Whats the volume of iron contained between the marks labeled 3.10cm & 3.50cm? The Volume of a cylinder is "pi"r^2 L. pi=3.1416,r=radius, L=length.

3-Using Scientific Notation Convert:
a. 6.20 km to m
b. 2.54 cm to m
c. 1.98 ns to s
d. 5.23 "micro"g to g
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1- I got .2 g, but IDK if I did it right.
2- I think it's 2.82744 but w/ correct Significant Figures = 2.8
3- ? IDK

*Can someone please check to see if I'm on the right track & also explain the concepts to me. I would really appreciate it! Thanks! = )

Answer 1

1) [Conservation of Mass Problem]

m = mass of hydrogen gas

m = (5.6g + 15.0g) - 20.4g = 20.6 - 20.4 = 0.2g

You are correct.
______________

2) Volume of Cylindrical Section

V = volume of cylinder

v = πr²h = π(1.5)²(3.5 - 3.1) = π(2.25)(0.4) = 0.9π
_________________________

3-Using Scientific Notation Convert:
a. 6.20 km = 6.2*10³ m
b. 2.54 cm = 2.54*10^(-2) m
c. 1.98 nanoseconds = 1.98*10^(-9) seconds
d. 5.23 micrograms = 5.23*10^(-6) grams

Answer 2

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