#### Question

# Calculate the mass, in grams of H2 needed to react with 25.0g Cl2?

I need help with a couple problems, please explain, thank you!

#1) For the reaction: H2(g) + Cl2(g) → HCl(g) (is this balanced?) calculate the mass, in grams of H2 needed to react with 25.0g Cl2.

The Answer is 0.711

Please explain the steps

#2) A 10.0 ml sample of an unknown HCl solution requires titration with 15.0ml of 0.1MNaOH solution to reach equivalent point. What is the concentration of the HCl solution? HCl + NaOH → H2O + NaCl

A.150

B.none of these

C.0.15

D.0.00015

#### Answers

1) H2(g) + Cl2(g) →2 HCl(g) is more correct (same number of H and Cl on both sides)

from this equation, one mole of Cl2 reacts with one mole of H2

a chlorine molecule is made up of two chlorine atoms, each having a molar mass of 35.5 g/mol, meaning the molecule has a mass of 70.1 g/mol

we convert the given 25 g to moles

moles = mass/molar mass

=25/70.1

=0.36 moles

since 1 mole of hydrogen reacts with one mole of chlorine, we need 0.36 moles of hydrogen. but how many grams is that. again, we use the equation. hydrogen has a mass of 1g/mol, so the molecule is 2g/mole (since its diatomic)

moles = mass/molar mass

0.36 = mass/2

mass = 2*0.36 = 0.72 g

its slightly off from your answer, but thats because i did quite a bit of rounding (im using the crappy windows xp calculator), but you get the gist of it.

#2)

one mole of HCl reacts with one mole of NaCl. the equivalence point is reached when all the HCl has been neutralised. that is, all of it has been consumed.

15mL of 0.1 M NaOH means there are a total of

0.015 * 0.1 = 0.0015 moles of NaOH

one mole of HCl reacts with one mole of NaCl, so you need 0.0015 moles of HCl

we have 10mL = 0.01L of the stuff, and since

molarity = moles/volume

=0.0015/0.01

=0.15 moles per liter or 0.15M

moles Cl2 = 25.0 g ( 1 mol / 70.906 g)= 0.352

the balanced equation is

H2 + Cl2 = 2 HCl

the ratio between H2 and Cl2 is 1 : 1

moles H2 required = 0.352

mass H2 = 0.352 mol x 2.016 g/mol=0.710 g

moles NaOH = 0.0150 L x 0.1 M = 0.00150

the balanced equation is

NaOH + HCl = NaCl + H2O

the ratio between NaOH and HCl is 1 : 1

moles HCl = 0.00150

Molarity of HCl = 0.00150 mol/ 0.0100 L=0.15 M