# Streptomyces:Other Bits/Useful Molecular and Chemical Equations

## Useful Molecular and Chemical Equations

Formula Weight & Molecular Weight

Formula weight (FW) and molecular weight (MW) are calculated by summing the atomic weights (AW measured in atomic mass units, amu) of the individual atoms.

 e.g. where: C = 12.01amu H = 1.00amu Na = 35.45amu Cl = 22.99amu

Chemical Name Chemical Formula Weight

Benzene C6H6 MW = 6*12.01 + 6*1.00 = 78.06amu
Sodium Chloride NaCl FW = 22.99 + 35.45 = 58.44amu

The difference between formula weight and molecular weight depends on the compound. It is correct to refer to a compound such as Benzene having a molecular weight or formula weight. It is incorrect to refer to sodium chloride having a molecular weight as NaCl exists as an ionic compound (Na+ Cl-) not as a molecular compound. In this case it is more precise to refer to sodium chloride’s formula weight.

1 mole of atoms / molecules has a mass equal to the atomic / molecular weight in grams.

e.g. 1 mole (1mol) NaCl is the number of molecules in 58.44g of NaCl. (1mol NaCl = 58.44g)

Avogadro’s number is the number of atoms / molecules in 1 mole of any substance, which is equal to 6.02214x1023.

$n=\frac {m}{FW}$

Molarity – Molar Concentration

Molarity is the number of moles of solute per litre of solution.

e.g. 6 molar (6M) HCl is equal to 6 moles (6mol) of HCl per litre (L). (6M HCl = 6mol/L)

 Where: n = Number of moles m = Mass in grams (g) FW = Formula weight M = Molarity in mol/litre (mol/L) V = Volume in litres (L)
$M=\frac {n}{V}$

Based on the previous two equations:

 m = MVFW $M=\frac {m}{VFW}$

Primer Calculations

Primers are dissolved in sterile distilled water (sdH2O) to a concentration of 500pmol. Use one of the following to determine what volume of sdH2O to use:

 $\frac {\mu g*10^{6}}{500*MW}=\mu L$ $\frac {nmol}{0.5}=\mu L$ $\frac {pmol}{500}=\mu L$

Weight / mole Percentage

The percentage weight of an element in a compound is calculated using the atomic weight and formula weight.

Chemical Name Chemical Formula Weight

Hydrochloric acid HCl FW = 1.00 + 35.45 = 36.45amu

$\frac {AW}{FW}*100=%weight$

Percentage weight of Cl in HCl:

$Cl=\frac {35.45}{36.45}=97.26%$

Similarly, mole percentage is a ratio.

 Where: x = Number of atoms of the element T = Total number of atoms in the compound

$\frac {x}{T}*100=%mole$

Percentage mole of Cl in HCl:

$Cl=\frac {1}{2}*100=50%$

Density and Specific Gravity

Density is the mass of a substance per volume.

 Where: D = Density (g/cc) m = Mass in grams (g) v = Volume in cubic centimetres (cc) SG = Specific Gravity

Specific gravity is a unitless ratio, so for all purposes; SG ≡ D. Cubic centimetres are equivalent to millilitres; cc ≡ mL.

 $D=\frac {m}{v}$ $SG=\frac {D_1}{D_2}$

D2 = Density of H2O @ 4°C = 1.00g/cc

Molarity, Specific Gravity and Percentage Composition

Calculating Molarity from specific gravity and percentage composition:

Chemical Name Formula weight Percentage Composition Specific Gravity

Hydrochloric acid 36.45amu 37% 1.18

Percentage composition means xg of pure compound per 100g of solution, i.e. 37g/100ml = 37%. To calculate the molarity, the mass of pure compound is needed; however the solution’s specific gravity needs to be taken into account, and the volume; which we’ll take to be 1L.

 m = SG * v * %composition $M=\frac {m}{VFW}$

Where 1L = 1000cc For HCl:

$m=\frac {1.18*1000*37}{100}=436.6g$

Therefore:

$M=\frac {436.6}{1*36.45}=11.97mol/L$