How many grams of solid sodium cyanide should be added to 1.00 L of a 0.119 M hydrocyanic acid solution to prepare a buffer with a pH of 8.809


Answer 1


1.62 g


Given that:

Concentration of HCN = 0.119 M

Assuming the ka 4.00 × 10⁻¹⁰

The pKa of  HCN (hydrocyanic acid)  = -log (Ka)

= - log ( 4.00 × 10⁻¹⁰)

= 9.398

pH of buffer = 8.809

Using Henderson Hasselbach equation:

pH = pKa + log ([conjugate\  base ])/(acid)

pH = pKa + log ([CN^-])/([HCN])

8.809 = 9.398 +log ([CN^-])/([HCN])

log ([CN^-])/([HCN])= 8.809 - 9.398

log ([CN^-])/([HCN])= -0.589

([CN^-])/([HCN])= 0.2576

[CN^-] = 0.2576[HCN]

[CN^-] = 0.2756 (0.119) L

[CN^-] = 0.033 M

The amount of NaCN (sodium cyanide) is calculated as follows:

= 1.00 L * (0.033 \ mol \ NacN )/(1 \ L ) * (49.01 \ g)/(1 \ mol \ of \ NacN)

= 1.62 g

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Aqueous hydrochloric acid HCl will react with solid sodium hydroxide NaOH to produce aqueous sodium chloride NaCl and liquid water H2O. Suppose 9.84 g of hydrochloric acid is mixed with 3.1 g of sodium hydroxide. Calculate the maximum mass of water that could be produced by the chemical reaction. Round your answer to 2 significant digits.



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In a reaction, the reactants are usually not present in exactstoichiometric amounts, that is, in the proportions indicated by the balanced equation. Frequently a large excess of one reactant is supplied to ensure that the more expensive reactant is completely converted to the desired product. Consequently, some reactant will be left over at the end of the reaction. The reactant used up first in a reaction is called the limiting reagent, because the maximum amount of product formed depends on how much of this reactant was originally present. When this reactant is used up, no more product can be formed.

A chemist designs a galvanic cell that uses these two half-reactions:O2 (g) + 4H+(aq) + 4e− → 2H2O (l) Eo =+1.23V
Zn+2 (aq) + 2e− → Zn(s) Eo=−0.763V

Answer the following questions about this cell.

Write a balanced equation for the half-reaction that happens at the cathode.
Write a balanced equation for the half-reaction that happens at the anode.
Write a balanced equation for the overall reaction that powers the cell. Be sure the reaction is spontaneous as written. Do you have enough information to calculate the cell voltage under standard conditions


Answer: The reaction is spontaneous and there is not enough information to calculate the cell voltage.


The substance having highest positive E^o reduction potential will always get reduced and will undergo reduction reaction.

Oxidation reaction occurs at anode and reduction reaction occurs at cathode.

  • For a:

The half reactions for the cell occurring at cathode follows:

O_2(g)+4H^+(aq)+4e^-\rightarrow H_2O(l);E^o_(cathode)=+1.23V

  • For b:

The half reactions for the cell occurring at anode follows:

Zn(s)\rightarrow Zn^(2+)+2e^-;E^o_(anode)=-0.763V    ( × 2)

  • For c:

The balanced equation for the overall reaction of the cell follows:

O_2(g)+4H^+(aq)+2Zn(s)\rightarrow H_2O(l)+2Zn^(2+)(aq)

For the reaction to be spontaneous, the Gibbs free energy of the reaction must come out to be negative.

Relationship between standard Gibbs free energy and standard electrode potential follows:

\Delta G^o=-nFE^o_(cell)

For a reaction to be spontaneous, the standard electrode potential must be positive.

To calculate the E^o_(cell) of the reaction, we use the equation:


Putting values in above equation, we get:


As, the standard electrode potential of the cell is coming out to be positive, the reaction is spontaneous in nature.

  • To calculate the EMF of the cell, we use the Nernst equation, which is:

E_(cell)=E^o_(cell)-(0.059)/(n)\log ([Zn^(2+)]^2)/([H^(+)]^4* p_(O_2))

As, the concentrations and partial pressures are not given. So, there is not enough information to calculate the cell voltage.

Hence, the reaction is spontaneous and there is not enough information to calculate the cell voltage.

The mass of a water balloon is 2 kilograms. The speed that the water ballon is traveling when it hits the ground is 20 meters/second.What is the total kinetic energy of a water balling that hits the ground after it is dropped from a balcony ?



400 Joules


From the question,

The total kinetic energy of the water balling when hits the ground is given as

K.E = 1/2mv².................. Equation 1

Where K.E = Kinetic Energy of water ballon, m = mass of water balloon, v = velocity of water ballon

Given: m = 2 kilograms, v = 20 meters/second.

Substitute these values into equation 1

K.E = (2×20²)/2

K.E = 2×400/2

K.E = 400 Joules

The chemical equation for this reaction is Ca + O2→CaO. What is the product, or what are the products, of this reaction?


Answer: Cao2

Explanation: the reason for this is because the chemical reactions on both sides should be equal and since the O has a 2 the O should have 2 on the other equation.

What is the density of iron if it crystallizes in a body-centered cubic unit cell with an edge length of 287 pm



Density = 7.87g/cm^3


Density is the ratio of mass of the given object to the volume of the object, in this question iron is the given object, then we make use of atomic number of iron


Length= 287pm = 287*10^-10cm

Atomic mass of Fe= 56.0u

Z=2(for body centered cubic unit cell)

Avogadro number (N 0)=6.022× 10^23

Density= ZM/a^3 × N


Z= body centered cubic unit cell

Then substitute

N= Avogadro's number


Density = (2× 56)/(287*10^-10cm)^3 × (6.022 × 10^23)

Density = 7.87g/cm^3

Final answer:

The density of iron in a body-centered cubic unit cell can be calculated using the mass and volume of the unit cell.


The density of iron can be calculated using the formula: density = mass/volume. To determine the mass of the unit cell, we need to know the molar mass of iron and the number of atoms in the unit cell. The molar mass of iron is 55.845 g/mol, and there are two iron atoms in the body-centered cubic unit cell of iron. The volume of the unit cell can be calculated using the formula: volume = (edge length)^3.

Putting these values into the formula, we get:

density = (2 * 55.845 g/mol) / ((287 pm)^3)

Converting the edge length to meters (1 pm = 1e-12 m) and calculating, we find that the density of iron is approximately 7.86 g/cm³.

Learn more about density of iron here:


What are the starting substances (molecules) in a chemical equation called?



A chemical reaction is the process in which atoms present in the starting substances rearrange to give new chemical combinations present in the substances formed by the reaction. These starting substances of a chemical reaction are called the reactants, and the new substances that result are called the products.