Flashcards in 1.2 Amount of substance Deck (17)

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1

## List the formulae you can use to find the no. of moles.

###
n=m/Mr moles=mass/molecular mass

n=cv moles=concentration (in moldm^-3) x volume (in dm^3)

2

## What is the number of particles representing one mole?

### 6.02x10^23

3

##
Empirical Formula:

"A compound is found to contain 23.3% magnesium, 30.7% sulfur and 46.0% oxygen. What is the empirical formula of this compound?"

###
Mg S O

23.3% 30.7% 46.0%

(Divide by Mr)

23.3/24.3 30.7/32.1 46.0/16.0

=0.959 =0.956 =2.875

(Divide by smallest value)

(/0.956) (/0.956) (/0.956)

= 1 =1 =3

\ | /

MgSO₃

4

## What is the ideal gas equation?

###
pV= nRT

Where:

p = pressure in kPa

V = volume in m^3

n = moles of gas present

R = the gas constant (8.31 JK^-1mol^-1)

T = The gas' temperature in kelvin.

5

##
Isotopic abundance:

If a sample of boron is composed of 19.9% atoms with an Ar of 10, and 80.1% atoms with an Ar of 11, calculate the atomic weight of boron.

###
(19.9x10) + (80.1x11) / 100

= 10.8 (1dp)

6

## How to remember which formulae to use:

###
FOR SOLUTIONS: n=cv

FOR SOLIDS: n=m/Mr

FOR GASES:pV=nRT

7

##
Calculate the volume of 0.50 moldm^-3 nitric acid required to react completely with 5 g of lead (II) carbonate.

PbCO3 + 2HNO3 --> Pb(NO3)2 + CO2 + H2O

###
n=m/Mr

Moles of PbCO3 = 5/267 = 0.0187

1:2 ratio- moles of HNO3 = 0.0187 x 2 = 0.0375

v=n/c

Volume of HNO3 = 0.0375/0.5 = 0.075dm^3

(0.075x1000=75cm^3 if answer needs to be in cm^3)

8

## What's the formula for percentage yield?

### actual yield/maximum theoretical yield x 100

9

##
What is the atom economy for making hydrogen by reacting coal with steam?

C(s) + 2H2O(g) → CO2(g) + 2H2(g)

###
Answer = 8.3%

12 + (2x18) → 44 + (2x2)

(2x2)/(12+36) x 100 = 8.3%

10

## What is the formula for percentage uncertainty?

### (Absolute uncertainty/Value) x100

11

## What is the concentration of the solution if 2.5g of calcium carbonate is dissolved in water, and the solution made up to 0.5dm^3?

###
Answer = 0.0500moldm^-3

n=m/Mr = 2.5/40.1+12.0+(3x16.0) = 0.0250mol

c=n/v = 0.250/0.5 = 0.0500moldm-3

12

## Calculate the Mr of a substance with a weight of 90g when there are 5 moles of it, and suggest what the substance could be.

###
Answers = 18, water.

Mr = m/n = 90/5 = 18

Water has an Mr of 18.

13

## Deduce the formula of the compound lead (IV) oxide.

### PbO2

14

## What is the mass of sulphur in 1 tonne of H2SO4?

###
Answer = 327kg

1 mole of H2SO4 = 98.1g

1000000 grams in a tonne

No. of moles in a tonne = 1000000/98.1 = 10200mol

10200 x 32.1 = 327000g (327kg)

15

## 25cm^3 of a solution of 0.1moldm^-3 NaOH reacts with 50cm^3 of a solution of hydrochloric acid. What is the molarity of the acid?

###
Answer = 0.05moldm^-3

NaOH + HCl → NaCl + H2O

Moles of NaOH: n=cv = 0.1 x (25/1000) = 0.0025mol

∴ There are 0.0025 moles of HCl

c=n/v = 0.0025/(50/1000) = 0.05moldm^-3

16

## How many moles of NaCl are there in 25cm^3 of a 50gdm^-3 solution?

###
n=m/Mr = 50 / 23.0+35.5 = 0.855moldm^-3

n=cv = 0.855 x (25/1000) = 0.0214mol

17