### salt water chemistry

```living with the lab
salt water mixtures
the world’s oceans contain an average of about 3.5% salt by weight
(35 grams of salt for every kilogram of seawater)
Image credit: NASA/GSFC/JPL-Caltech
living with the lab
concentrations used for fishtank project
we will calibrate our salinity sensor using small NaCl concentrations
• 0.00% weight NaCl – we will actually use deionized water (DI water)
• 0.05% weight NaCl
• 0.10% weight NaCl
• 0.15% weight NaCl
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why not just use tap water?
• standard tap water contains dissolved ions such as sodium, calcium, iron,
copper and bromide
• the electrical resistivity of tap water can vary widely (15kΩ-cm is typical)
• ions have been removed from DI water resulting in a much higher
resistivity (18MΩ-cm is typical)
Did you notice the strange units on resistivity (Ω-cm)? The electrical resistance R of a
body (measured in Ω) is related to the electrical resistivity ρ (measured in Ω-cm) as

=
∙

A = cross sectional area
L = length over which resistance is measured
ρ = resistivity which is a material property
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%  =

∙ 100% =
∙ 100%
+ 2
+ 2
calculating percent weight
A mixture contains 19 grams of water and 1 gram of NaCl. What is the weight
percent of NaCl?
%  =

ℎ
∙ 100%
∙ 100% =
+ 2
ℎ
=
∙
∙ 100%
∙  + 2  ∙
=

∙ 100%
+ 2
=
1
1
∙ 100%
∙ 100% =
20
1  + 19
= 9.81

2
%  = 5%
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Class Problem If you add 9.5 grams of NaCl to 5 gallons of water, what weight
percent of salt will the mixture contain?
Useful conversion factors:
Density of water = 2  = 1000 kg/m3 = 1 g/cm3 = 1 kg/L at 4oC (maximum density)
1 cm3 = 1 cc = 1 ml
1 L = 0.001 m3
1 gallon = 3.7853 L
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living with the lab
Example
How much salt would you need to add to 2L of water to have a
concentration of 3.5 wt% NaCl?
unknown
%  =
3.5%

∙ 100%
+ 2
unknown
3.5% =
2  = 2 ∙
1
= 2

mass of 2L of water

∙ 100%
+ 2
0.035  + 2 =
0.035 ∙  +0.07 =
1 − 0.035 ∙  = 0.07
=
0.07
= 0.0725 = 72.5
1 − 0.035
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recall the reactions at the electrodes
5V
e-
e-
10 kΩ
anode – oxidation
cathode – reduction
(loss of electrons)
e-
(gain of electrons)
e-
Cl-
Cl-
Na+
Cl2
Cl-
Na+
Cl-
Cl
Cl-
ClCl-
Na+
Na+
Cl
Cl-
reduction occurs at the
negatively charged cathode:
Na+
Na+
Na+
Cl-
Na+
ion migration
OHee-
2 2 () + 2 − → 2  + 2− ()
H
H2O
H
H2O
OH-
ClNa+
Cl-
Cl-
Na+
Na+
Na+
Na+ is a spectator ion
oxidation occurs at the
positively charged anode:
2  −  → 2  + 2 −
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useful information for reaction calculations

Atomic Weight of Na = 22.99
Atomic Weight of Cl = 35.45

Atomic Weight of NaCl = 58.44
6.022 x 1023

1 Coulomb = 6.24 x 1018 electrons
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Atomic Weight of NaCl = 58.44
6.022 10 23

Class Problem Assume you have 5 gallons of water to which you add salt to
create a mixture with 0.2 wt% NaCl. Determine:
(a) the mass of the water
(b) the mass of the salt
(c) the number of moles of NaCl
(d) the number of Cl- ions
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1 Coulomb = 6.24 x 1018 electrons
1A=
1

Example
If a constant current of 0.1mA passes through the probes of the
conductivity sensor, how many H2 gas molecules would be formed over a 1 minute period?
HINT: Use the definition of an amp and a Coulomb along with the chemical reaction at the cathode.
2 2 () + 2 − → 2  + 2− ()
two electrons pass through the circuit for each H2 gas molecule formed
First, find the number of electrons that pass through the sensor per second.
−

= 0.1 ∙
∙

1000

6.24 10 18  −

= 6.24 10
∙

14
−

Now, find the number of H2 molecules over a 1 minute period.
2  = 6.24 10
14
1 2
− 60
∙
∙ 1  ∙
= 1.87 10
2 −

16
2
10
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